Fix PHP 8.1 "strlen(null)" and preg_match() exceptions which block rendering "Browse Subscribers" 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, preg_match() does not accept passing null as $subject parameter since PHP 8.1.

Closes T15381

Test Plan: Applied these two changes (on top of D25179 and D25213), and on the task creation page, after clicking the magnifier icon in the "Subscribers" field, the "Browse Subscribers" overlay dialog got rendered.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15381

Differential Revision: https://we.phorge.it/D25214