Fix PHP 8.1 "strlen(null)" exception which blocks rendering Phortune account page

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 T15363

Test Plan: Applied this change and "Default Account" page `/phortune/account/1/details/` 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: T15363

Differential Revision: https://we.phorge.it/D25201

src/applications/phortune/xaction/PhortuneAccountNameTransaction.php

···
     $old = $this->getOldValue();
     $new = $this->getNewValue();
 
-    if (strlen($old) && strlen($new)) {
+    if (phutil_nonempty_string($old) && phutil_nonempty_string($new)) {
       return pht(
         '%s renamed this account from %s to %s.',
         $this->renderAuthor(),