Followup to rARC99e57a70. 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.

Run static code analysis.