fix(xml): render example with oneOf/anyOf

[tim-lai]

• test(e2e): render xml example with oneOf/anyOf

Description

Update and simplify getXmlSampleSchema logic by removing the "catchall" return null case. return null would destroy the original schema object so no example could be generated. Note that this logic has been present since the original release of SwaggerUI@3, so there may be a need to add further logic to compensate for the previous "catchall" case for return null.

Motivation and Context

fixes #8205

How Has This Been Tested?

Existing tests continue to pass. New e2e tests added.

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.