fix the multiple implicit nullable variable in php 8.4

Closes #3516167

src/Plugin/RabbitHoleBehaviorPlugin/AccessDenied.php

@@ -20,7 +20,7 @@ class AccessDenied extends RabbitHoleBehaviorPluginBase {
   /**
    * {@inheritdoc}
    */
-  public function performAction(EntityInterface $entity, Response $current_response = NULL) {
+  public function performAction(EntityInterface $entity, ?Response $current_response = NULL) {
     throw new AccessDeniedHttpException();
   }