Fix PHP 8.1 "strlen(null)" exception on Diffusion repo URIs page after repo 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.

```
ERROR 8192: strlen(): Passing null to parameter freebsd#1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/repository/storage/PhabricatorRepository.php:348]
```

Closes T15658

Test Plan: Create an empty new Git repository, go to its URIs management page.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15658

Differential Revision: https://we.phorge.it/D25454

Author: aklapper

src/applications/repository/storage/PhabricatorRepository.php

@@ -345,12 +345,12 @@ public function getCloneName() {
 
     // Make some reasonable effort to produce reasonable default directory
     // names from repository names.
-    if (!strlen($name)) {
+    if (!phutil_nonempty_string($name)) {
       $name = $this->getName();
       $name = phutil_utf8_strtolower($name);
       $name = preg_replace('@[ -/:->]+@', '-', $name);
       $name = trim($name, '-');
-      if (!strlen($name)) {
+      if (!phutil_nonempty_string($name)) {
         $name = $this->getCallsign();
       }
     }