Fix strlen(null) PHP 8.1 error when pushing into a fresh repository

Summary:
Doing a git push into a fresh repository over https fails with a PHP 8.1 strlen(null) error.

This fixes it.

Fixes T15569

Test Plan:
* Create an empty repository on a PHP 8.1 Phorge instance
* Clone it
* Add some data
* git push

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15569

Differential Revision: https://we.phorge.it/D25361

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -1133,7 +1133,7 @@ final class DiffusionCommitHookEngine extends Phobject {
       ->setHookWait(phutil_microseconds_since($hook_start));
 
     $identifier = $this->getRequestIdentifier();
-    if (strlen($identifier)) {
+    if ($identifier !== null && strlen($identifier)) {
       $event->setRequestIdentifier($identifier);
     }