Merge branch '5.4' into 6.4

* 5.4:
  Make more nullable types explicit
  Add more explicit nullable types for default null values

Handler/BatchHandlerInterface.php

@@ -23,7 +23,7 @@ interface BatchHandlerInterface
      * @return mixed The number of pending messages in the batch if $ack is not null,
      *               the result from handling the message otherwise
      */
-    // public function __invoke(object $message, Acknowledger $ack = null): mixed;
+    // public function __invoke(object $message, ?Acknowledger $ack = null): mixed;
 
     /**
      * Flushes any pending buffers.