Fix various potential PHP 8.1 "strlen(null)" exceptions

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.

All lines changed in this patch had Parameter #1 $string of function strlen expects string, string|null given reported by PHPStan. Thus these should be safe to replace with phutil_nonempty_string as no calls care about the actual strlen() return value (length of a string).

Test Plan: Run static code analysis via phpstan analyse -l 9

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25778