fix: send null body for DELETE requests when no body is provided

[sourabhgithubcode]

Summary

Fixes #18

Http.delete failed when called without a body or json argument because the body defaulted to "" (empty string). The request library then sent a Content-Length: 0 header, causing many REST APIs to reject DELETE requests with 400/411 errors.

Root cause: The body fallback used "" instead of null for all non-GET methods:

// before
body = (operation === 'get') ? (null) : (self.args['body'] || ""),

// after
body = (operation === 'get') ? (null) : (self.args['body'] || null),

When body is null, the request library sends no body and omits Content-Length, which is the correct behavior for a bodyless DELETE.

Changes

• src/Nodes/FunctionNodes/HttpDriver/HttpNode.js — change default body fallback from "" to null
• src/Nodes/FunctionNodes/HttpDriver/test/http.js — add two DELETE tests:
  • DELETE without a body succeeds (regression test for Http.delete fails is body / json parameter is not passed #18)
  • DELETE with a json body correctly sends the payload

Test plan

• npm test — all existing tests pass
• New DELETE tests pass against httpbin.org