Fix PhabricatorAuthCSRFEngine.php strncmp(null) PHP 8.1 error

Summary:
Update PhabricatorAuthCSRFEngine.php such that it doesn't fall over when provided with a null CSRF token under PHP 8.1

Fixes T15654

Test Plan: Do a POST request to phorge.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15654

Differential Revision: https://we.phorge.it/D25449