fix: Support for generating enums when enums definition has null value

[DanDeMicco]

Changes

What does this PR change? Link to any related issue(s). #1872

How to Review

How can a reviewer review your changes? What should be kept in mind for this review?

Checklist

• [x] Unit tests updated
• [ ] docs/ updated (if necessary)
• [x] pnpm run update:examples run (only applicable for openapi-typescript)