Fix PHP 8.1 "strlen(null)" and "preg_match()" exceptions which block repository creation

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, preg_match() does not accept passing null as $subject parameter since PHP 8.1.

Closes T15337

Test Plan: Applied these four changes and /diffusion/4/manage/ finally rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15337

Differential Revision: https://we.phorge.it/D25182