check null pointer message

[marcodifalco]

In this method

 @Override
    public void outgoingBroadcast(Object message) {
        logger.trace("{} outgoingBroadcast {}", topic, message);

        // TODO: Prevent message round trip.

        producer.send(new ProducerRecord<String, String>(topic, message.toString()));
    }

There are not a check if 'message' is null.
Without this check it's impossible use the lib.
I make override class, and override method, and i add
if (message!= null)``