[php][php-nextgen] fix return type if empty and non-empty responses are mixed

[JulianVennen]

This PR fixes an issue when you have an endpoint that either returns a response with data or an empty response (e.g., empty response on success but an error message on error, or the other way around). The return type would be annotated with the non-empty response types. If an empty response was encountered, null would be returned which throws an error because that's not a valid return type.

I added null to the possible return types if any of the responses have no schema.

Fixes #21359.

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