Fix PHP 8.1 "strlen(null)" exception which blocks deleting Phriction document

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 T15419

Test Plan:
Applied this changes and page at /w/ correctly rendered saying Document Deleted - This document has been deleted. You can edit it to put new content here, or use history to revert to an earlier version.

Also test creating it again, and updating it again: it still works.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15419

Differential Revision: https://we.phorge.it/D25243