Fix PHP 8.1 "strlen(null)" exception[s] which block rendering "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.

Closes T15380

Test Plan: Applied these two changes (on top of D25179) and on the task creation page, after clicking the magnifier icon in the "Tags" field, the "Browse Projects" 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: T15380

Differential Revision: https://we.phorge.it/D25213