Fix PHP 8.1 null parameter exceptions which block rendering the "Browse Projects" overlay dialog

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Similarly, passing null to the $haystack parameter of strpos() is deprecated in PHP 8.1.

Similarly, passing null to the $string parameter of ltrim() is deprecated in PHP 8.1.

Closes part of T15335

Test Plan: Applied these four changes in Phorge (plus the one change in D25180 in Arcanist) 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/D25179