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