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.

This patch is a followup on top of D25814.

Refs T15935

Test Plan:

• Try to view a task in the browser
• Try to run ./bin/auth recover on the CLI
• Try to browse projects in the browser
• Try to create a new project without a name in the browser

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/D25816