Do not pass a null string to mb_convert_case() for PHP 8.1 compatibility

Summary:
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