[php-nextgen] Use conditional access for enumref query params

[JulianVennen]

Query parameters in the php-nextgen generator are passed to the ObjectSerializer::toQueryValue method. For enum refs (which use native PHP enums) the value property, which contains the backing string or number, is passed instead. This causes a PHP warning when the parameter is null. To prevent that, I added a ? which causes the resulting value to be null if the parameter itself is null.

This is not required for header or path parameters, as these are already inside a null check.

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[JulianVennen]

The typescript test failed due to a network error, I don't think that has anything to do with this PR ;)

[wing328]

agreed https://github.com/OpenAPITools/openapi-generator/actions/runs/12674809528/job/35324360920?pr=20418 failure is not related to this change.