Bugfix in RdKafkaProducer:flush() method

Avoid TypeError in `RdKafkaProducer::flush()`

Author: alexbusu-motork

pkg/rdkafka/RdKafkaProducer.php

@@ -121,5 +121,7 @@ public function flush(int $timeout): ?int
         if (method_exists($this->producer, 'flush')) {
             return $this->producer->flush($timeout);
         }
+
+        return null;
     }
 }