Fix PHP 8.1 "strlen(null)" exception in Feed after renaming EditEngine

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/transactions/xaction/PhabricatorEditEngineNameTransaction.php:17]
```

Closes T16515

Test Plan: Unclear, likely rename some EditEngine (see e.g. http://phorge.localhost/transactions/editengine/) and go to http://phorge.localhost/feed/transactions/ and export data

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16515

Differential Revision: https://we.phorge.it/D26782

src/applications/transactions/xaction/PhabricatorEditEngineNameTransaction.php

···
   }
 
   public function getTitle() {
-    if (strlen($this->getOldValue())) {
+    if (phutil_nonempty_string($this->getOldValue())) {
       return pht(
         '%s renamed this form from %s to %s.',
         $this->renderAuthor(),