[php][php-nextgen] Fix nullability when multiple response types are possible

[JulianVennen]

Fixes #22817

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---

Summary by cubic

Fixes nullability handling for PHP Nextgen when an operation can return multiple types. Generates "type1|type2|null" instead of invalid "?type1|type2", and keeps "?T" for single types; addresses #22817.

• Bug Fixes
  • For endpoints with multiple response schemas plus an empty response, return types now use unions with null (e.g., integer|string|null). Single-type responses remain "?T".
  • Added a test and sample spec (issue_22817.yaml) to assert correct generation and prevent "?union" output.

^{Written for commit 56d373d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files