[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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@jebentier @dkarlovi @mandrean @jfastnacht @ybelenko @renepardon

[kocsismate]

@wing328 I have bumped into an error when a response has a nullable union return type: then the native return type will include the following: ?A|B which is invalid syntax. As far as I remember, the same happens with other symbols (e.g. parameters maybe?).

[JulianVennen]

Can you create an issue with a minimal OpenAPI spec to reproduce that?

[kocsismate]

Thanks! Done: #22817