Fix PHP 8.1 "strlen(null)" exception which blocks Space creation

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 T15340

Test Plan: Applied this change and page `/S3` rendered 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: T15340

Differential Revision: https://we.phorge.it/D25185

src/applications/spaces/xaction/PhabricatorSpacesNamespaceNameTransaction.php

@@ -15,7 +15,7 @@ final class PhabricatorSpacesNamespaceNameTransaction
 
   public function getTitle() {
     $old = $this->getOldValue();
-    if (!strlen($old)) {
+    if (!phutil_nonempty_string($old)) {
       return pht(
         '%s created this space.',
         $this->renderAuthor());