Fix PHP 8.1 "strlen(null)" exception which blocks rendering a File page when Alt Text was altered

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 T15342

Test Plan:
Applied these two changes on top of D25186.
Added an Alt Text to a file; page of the File now rendered correctly in web browser.
Removed Alt Text from a file; page of the File now rendered correctly in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15342

Differential Revision: https://we.phorge.it/D25187