[php] Fix PHP generator validation for nullable required properties

[achton]

Fix PHP generator bug where nullable-but-required model properties were always flagged as invalid when set to null. Update validation/docblocks so explicit null assignments are honored. Closes #21497.

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

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[wing328]

can we also apply the same update here to handle both required and optional properties?

[wing328]

can we also apply the same update here to handle both required and optional properties?

[wing328]

I'll address those comments with a separate PR

UPDATE: merged #22307

[wing328]

FYI. Merged another PR (#22310) to refactor the template a bit