Add nullable type to types that will throw deprecation errors in PHP 8.4

[KorvinSzanto]

This PR prevents the upcoming php 8.4 deprecation errors caused by nullable parameters being typed without a nullable type.

Interestingly, this also uncovered a couple places where constructors had parameters with default values before parameters without default variables, which is also deprectated. See eb938a6 and c1a6a57 for the specific backwards-compatible fixes for that

Fixer can be seen here: https://gist.github.com/KorvinSzanto/13eed5210561f6d090b159eae65b93a6
See https://wiki.php.net/rfc/deprecate-implicitly-nullable-types for more info

[mlocati]

Against 9.2.x? If so, will the minimum requirement of 9.2 be raised to PHP 8?
Otherwise we could use ?something instead of something|null

[KorvinSzanto]

Otherwise we could use ?something instead of something|null

That's fair, I've updated the PR

[aembler]

This is going to be an absolute nightmare to get into 10.x and 9.3.x as it is. I honestly don't know how we're going to do it. I'd strongly recommend this going into 9.3.

[aembler]

Never mind, I see that you have a separate PR for 10. Thank you!