Fix PHP 8.1 "strlen(null)" exception on /diffusion/external/

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/diffusion/controller/DiffusionExternalController.php:62]
```

Closes T16119

Test Plan: Go to http://phorge.localhost/diffusion/external/

Reviewers: O1 Blessed Committers, chris

Reviewed By: O1 Blessed Committers, chris

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16119

Differential Revision: https://we.phorge.it/D26099

src/applications/diffusion/controller/DiffusionExternalController.php

···
 
     if (empty($commits)) {
       $desc = null;
-      if (strlen($uri)) {
+      if (phutil_nonempty_string($uri)) {
         $desc = pht('"%s", at "%s"', $uri, $id);
       } else {
         $desc = pht('"%s"', $id);