Fix implicitly nullable parameter declarations for PHP 8.4

Summary:
Followup to rPdb61eb20 and rPf3d49f74. This patch should cover all remaining issues now that PHPStan covers it (instead of the previous trial-and-error approach).

Implicitly nullable parameter declarations are deprecated in PHP 8.4:
https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

The proposed syntax was introduced in PHP 7.1 and Phorge requires PHP 7.2 now.

Test Plan: Run static code analysis.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25832