Fix PHP 8.1 "strlen(null)" exception rendering a File's default Alt text

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.

Refs T15936

Test Plan: Read the code; compare to D25186.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15936

Differential Revision: https://we.phorge.it/D25817

src/applications/files/controller/PhabricatorFileViewController.php

@@ -316,7 +316,7 @@ final class PhabricatorFileViewController extends PhabricatorFileController {
     }
 
     $default_alt = $file->getDefaultAltText();
-    if (strlen($default_alt)) {
+    if (phutil_nonempty_string($default_alt)) {
       $finfo->addProperty(pht('Default Alt Text'), $default_alt);
     }