fix(enum-values): access optional prop child props

[darkbasic]

Changes

Fixes https://github.com/openapi-ts/openapi-typescript/issues/2138 and https://github.com/openapi-ts/openapi-typescript/issues/2140

How to Review

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Checklist

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