Nullable unions without enum

[fantapop]

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you written unit tests?
• Have you written unit tests that cover the negative cases (i.e.: if bad data is submitted, does the library respond properly)?
• This PR is associated with an existing issue?

Closing issues

closes #668

Related Issues

relates to #479

If this is a new feature submission:

• Has the issue had a maintainer respond to the issue and clarify that the feature is something that aligns with the goals and philosophy of the project?

Potential Problems With The Approach

One issue with this approach is that typescript when compiled with strictNullChecks, appears to not allow enums with null value. This feels like a typescript bug to me but I haven't been able to track it down.

Test plan
I updated existing unit tests to cover the new behavior. I additionally experiment with code generated with the new changes when possible. Code generation for oneOfs is not currently working with typescript-fetch generator.

[WoH]

LGTM, thanks for contributing!