allow None in consumer_group
allow None in consumer_group

wikimedia/operations-debs-python-pykafka,thedrow/samsa,benauthor/pykafka,wikimedia/operations-debs-python-pykafka,thedrow/samsa,benauthor/pykafka,thedrow/samsa,benauthor/pykafka,wikimedia/operations-debs-python-pykafka