Fix PHP 8.1 "strlen(null)" exception when DarkConsole is enabled

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.

This is not visible in the web user interface, only in the PHP error log.

Closes T15344

Test Plan: Applied this change, on top of D25165 locally applied, and no stacktrace is printed anymore in the PHP error log.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15344

Differential Revision: https://we.phorge.it/D25188