Fix PHP 8.1 "strlen(null)" exception initializing Diffusion repository without path access permissions

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 T15459

Test Plan: After applying this change, going to `/diffusion/17/manage/`, the actual error (`cannot mkdir /var/repo/17: Permission denied`) is propagated and displayed in the "Working Copy Status" section, instead of being hidden by a `strlen()` exception. See T15459 for details.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15459

Differential Revision: https://we.phorge.it/D25283

src/applications/repository/storage/PhabricatorRepository.php

@@ -1160,7 +1160,7 @@ final class PhabricatorRepository extends PhabricatorRepositoryDAO
    */
   public function getRemoteURIObject() {
     $raw_uri = $this->getDetail('remote-uri');
-    if (!strlen($raw_uri)) {
+    if (!phutil_nonempty_string($raw_uri)) {
       return new PhutilURI('');
     }