feat(ls): add validation for empty responses object

[glowcloud]

Adds validation for empty Responses Object. If the Responses Object has at least one non-specification extension field, there will be no validation error.

• no fields => error shown

responses:

• specification extension field only => error shown

responses:
   x-extension: 123

• incorrect HTTP Status Code => no error

responses:
   test:
      description: ok

• correct HTTP Status Code => no error

responses:
   '200':
      description: ok