Adds nullable enum integration test

[artem-zakharchenko]

:rocket: Why this change?

• Related issue seems to be fixed, but requires an integration test to be verified

:memo: Related issues and Pull Requests

• Closes #507

:white_check_mark: What didn't I forget?

• [x] To write docs
• [ ] To write tests
• [x] To put Conventional Changelog prefixes in front of all my commits and run npm run lint

[artem-zakharchenko]

I'm not sure if that's the best way to write such test. I haven't found an example of integration test in Dredd that would test multiple responses (with actual bodies).

[artem-zakharchenko]

@kylef, could you please look at this and verify if such test on Dredd side makes sense?