Do not pass a null string to mb_convert_case() for PHP 8.1 compatibility
Passing null to the $string parameter of mb_convert_case() is deprecated in PHP 8.1.
This is one of the exceptions which block rendering the "Browse Projects" overlay dialog.
Closes part of T15335
Test Plan: Applied this change in Arcanist (plus the four changes in D25179 in Phorge) and the Browse Projects overlay dialog finally rendered in web browser and listed existing projects.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15335
Differential Revision: https://we.phorge.it/D25180