Make nullable parameters explicit in generated entities

[derrabus]

Currently, EntityGenerator might generate methods with implicitly nullable parameters like this:

public function setOtherEntity(OtherEntity $other = null) {

However, this is deprecated in PHP 8.4 and we need to generate the code like this:

public function setOtherEntity(?OtherEntity $other = null) {

[derrabus]

ref #11624