Fix PHP 8.1 "strlen(null)" exception creating a Legalpad 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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/legalpad/xaction/LegalpadDocumentTitleTransaction.php:22]
```

Closes T16505

Test Plan:
* Create a Legalpad at http://phorge.localhost/legalpad/edit/
* Go to http://phorge.localhost/feed/transactions/

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16505

Differential Revision: https://we.phorge.it/D26766

src/applications/legalpad/xaction/LegalpadDocumentTextTransaction.php

···
   public function getTitle() {
     $old = $this->getOldValue();
 
-    if (!strlen($old)) {
+    if (!phutil_nonempty_string($old)) {
       return pht(
         '%s set the document text.',
         $this->renderAuthor());