Fix implicitly nullable parameter declarations for PHP 8.4

Summary:
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.

Note: This code change does *not* fix all occurrences but only allows passing ./bin/storage upgrade.

Refs T15935

Test Plan: Try to successfully run ./bin/storage upgrade with PHP 8.4

Reviewers: O1 Blessed Committers, chris

Reviewed By: O1 Blessed Committers, chris

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15935

Differential Revision: https://we.phorge.it/D25814