Fix PHP 8.1 "strlen(null)" exception for "/bin/auth refresh" without parameter

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/auth/management/PhabricatorAuthManagementRefreshWorkflow.php:37]
```

Test Plan: Run `../phorge/bin/auth refresh`: Get no `strlen()` error anymore but only expected output `Found 3 account(s) to refresh.` (or similar)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25583

src/applications/auth/management/PhabricatorAuthManagementRefreshWorkflow.php

@@ -34,7 +34,7 @@ final class PhabricatorAuthManagementRefreshWorkflow
         ));
 
     $username = $args->getArg('user');
-    if (strlen($username)) {
+    if (phutil_nonempty_string($username)) {
       $user = id(new PhabricatorPeopleQuery())
         ->setViewer($viewer)
         ->withUsernames(array($username))