Fixing static analysis issues

Fixing issue raised by phpstan

Author: JimTools

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;
     }
 }