[Bug]: Error getting offers with empty imported fleet

[jvstme]

Steps to reproduce

1. Create an SSH fleet
2. Add it to an export
3. Delete instances in the fleet (dstack fleet delete <name> -i ...).
4. Immediately after that, submit a run in the importer project.

Actual behaviour

$ dstack apply -b remote
Unexpected error: status code 500 when requesting 
http://localhost:3000/api/project/main/runs/get_plan. Check the server logs for backend issues, and
the CLI logs at (~/.dstack/logs/cli/latest.log) local CLI output

Expected behaviour

Run submitted successfully, possibly fails due to no capacity if there are no other fleets.

dstack version

master

Server logs

Traceback (most recent call last):
  File "/dstack/.venv/lib64/python3.10/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "/dstack/.venv/lib64/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/errors.py", line 187, in __call__
    raise exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/errors.py", line 165, in __call__
    await self.app(scope, receive, _send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 176, in __call__
    with recv_stream, send_stream, collapse_excgroups():
  File "/usr/lib64/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/_utils.py", line 82, in collapse_excgroups
    raise exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 178, in __call__
    response = await self.dispatch_func(request, call_next)
  File "/dstack/src/dstack/_internal/server/app.py", line 342, in log_http_metrics
    response: Response = await call_next(request)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 156, in call_next
    raise app_exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 141, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 176, in __call__
    with recv_stream, send_stream, collapse_excgroups():
  File "/usr/lib64/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/_utils.py", line 82, in collapse_excgroups
    raise exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 178, in __call__
    response = await self.dispatch_func(request, call_next)
  File "/dstack/src/dstack/_internal/server/app.py", line 288, in log_request
    response: Response = await call_next(request)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 156, in call_next
    raise app_exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/base.py", line 141, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/routing.py", line 714, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/routing.py", line 734, in app
    await route.handle(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/routing.py", line 288, in handle
    await self.app(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/routing.py", line 76, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/dstack/.venv/lib64/python3.10/site-packages/starlette/routing.py", line 73, in app
    response = await f(request)
  File "/dstack/.venv/lib64/python3.10/site-packages/fastapi/routing.py", line 301, in app
    raw_response = await run_endpoint_function(
  File "/dstack/.venv/lib64/python3.10/site-packages/fastapi/routing.py", line 212, in run_endpoint_function
    return await dependant.call(**values)
  File "/dstack/src/dstack/_internal/server/routers/runs.py", line 131, in get_plan
    run_plan = await runs.get_plan(
  File "/dstack/src/dstack/_internal/server/services/runs/__init__.py", line 356, in get_plan
    job_plans = await get_job_plans(
  File "/dstack/src/dstack/_internal/server/services/runs/plan.py", line 165, in get_job_plans
    fleet_model, instance_offers, backend_offers = await find_optimal_fleet_with_offers(
  File "/dstack/src/dstack/_internal/server/services/runs/plan.py", line 344, in find_optimal_fleet_with_offers
    candidate_fleet = fleet_model_to_fleet(candidate_fleet_model)
  File "/dstack/src/dstack/_internal/server/services/fleets.py", line 863, in fleet_model_to_fleet
    project_name=fleet_model.project.name,
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/attributes.py", line 566, in __get__
    return self.impl.get(state, dict_)  # type: ignore[no-any-return]
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/attributes.py", line 1086, in get
    value = self._fire_loader_callables(state, key, passive)
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/attributes.py", line 1121, in _fire_loader_callables
    return self.callable_(state, passive)
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/strategies.py", line 978, in _load_for_state
    return self._emit_lazyload(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/strategies.py", line 1079, in _emit_lazyload
    return loading.load_on_pk_identity(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/loading.py", line 694, in load_on_pk_identity
    session.execute(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/session.py", line 2365, in execute
    return self._execute_internal(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/session.py", line 2251, in _execute_internal
    result: Result[Any] = compile_state_cls.orm_execute_statement(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/orm/context.py", line 306, in orm_execute_statement
    result = conn.execute(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/base.py", line 1416, in execute
    return meth(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/sql/elements.py", line 523, in _execute_on_connection
    return connection._execute_clauseelement(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/base.py", line 1638, in _execute_clauseelement
    ret = self._execute_context(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/base.py", line 1843, in _execute_context
    return self._exec_single_context(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/base.py", line 1983, in _exec_single_context
    self._handle_dbapi_exception(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/base.py", line 2355, in _handle_dbapi_exception
    raise exc_info[1].with_traceback(exc_info[2])
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/base.py", line 1964, in _exec_single_context
    self.dialect.do_execute(
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/engine/default.py", line 945, in do_execute
    cursor.execute(statement, parameters)
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/dialects/sqlite/aiosqlite.py", line 172, in execute
    self._adapt_connection._handle_exception(error)
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/dialects/sqlite/aiosqlite.py", line 323, in _handle_exception
    raise error
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/dialects/sqlite/aiosqlite.py", line 149, in execute
    _cursor = self.await_(self._connection.cursor())
  File "/dstack/.venv/lib64/python3.10/site-packages/sqlalchemy/util/_concurrency_py3k.py", line 123, in await_only
    raise exc.MissingGreenlet(
sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_only() here. Was IO attempted in an unexpected place? (Background on this error at: https://sqlalche.me/e/20/xd2s)

Additional information

No response