Fix a PHP 8.1 deprecated use of strlen with a NULL argument in commit parser

Summary:
With PHP 8.1+ it is not possible to import a commit if the commiter field is not properly defined
Indeed, if the committer is not properly defined, strlen(null) is called, causing a deprecation warning, elevated to exception.
Using strlen() to check string validity is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Fix T15629

Test Plan:
- Push a commit to an observed subversion repository
- Import it via the phorge/bin/repository reparse COMMIT_ID --importing
- The commit should be properly imported and available in Diffusion

Reviewers: O1 Blessed Committers, Sten, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Sten, valerio.bozzolan

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

Maniphest Tasks: T15629

Differential Revision: https://we.phorge.it/D25423

src/applications/repository/worker/commitmessageparser/PhabricatorRepositoryCommitMessageParserWorker.php

@@ -36,7 +36,7 @@ abstract class PhabricatorRepositoryCommitMessageParserWorker
 
     $author = $ref->getAuthor();
     $committer = $ref->getCommitter();
-    $has_committer = (bool)strlen($committer);
+    $has_committer = phutil_nonempty_string($committer);
 
     $identity_engine = id(new DiffusionRepositoryIdentityEngine())
       ->setViewer($viewer)