fix: explicit nullable type php 8.4

Fix 8.4 deprecation warning

Author: bramstroker

src/Plugin/HandleQueueMessageRejectPlugin.php

@@ -28,7 +28,7 @@ public function aroundReject(
         callable $proceed,
         EnvelopeInterface $envelope,
         bool $requeue = true,
-        string $rejectionMessage = null
+        ?string $rejectionMessage = null
     ): void {
         if (!$rejectionMessage) {
             $proceed($envelope, $requeue, $rejectionMessage);