fixed deprecation notice

Author: websmurf

src/Extensions/Extension.php

@@ -13,7 +13,7 @@ interface Extension
      * @param EntityManagerInterface $em
      * @param Reader|null            $reader
      */
-    public function addSubscribers(EventManager $manager, EntityManagerInterface $em, Reader $reader = null);
+    public function addSubscribers(EventManager $manager, EntityManagerInterface $em, ?Reader $reader = null);
 
     /**
      * @return array