Fix headers in KafkaReceiver.php on line 64.

This issue can be reproduced if you use your package on one side and srooze enqueue on other side.

It fixes, because you use declare(strict_types=1):
[ErrorException]
Notice: Undefined property: RdKafka\Message::$headers

src/Messenger/KafkaReceiver.php

@@ -61,7 +61,7 @@ public function get(): iterable
 
                 $envelope = $this->serializer->decode([
                     'body' => $message->payload,
-                    'headers' => $message->headers,
+                    'headers' => $message->headers ?? [],
                     'key' => $message->key,
                     'offset' => $message->offset,
                     'timestamp' => $message->timestamp,