Fix: Use anyof and nullable: true for nullable unions w/refs

[fantapop]

Using the " anyOf with nullable: true" form has the benefit that the
openapi typescript-fetch generator already has support for this
construct.

All Submissions:

• 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?

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

None that I'm aware

Test plan

I modified the existing test which was relevant to ensure the correct values were being returned

[WoH]

I don't have a preference either way, so if this helps with client generation, LGTM.
Thank you!