Handle change in default groupId behavior in kafka consumer

Starting kafka 2.2.0 onwards, when group.id is not specified in the kafka consumer config, it is set as null, instead of an empty string. Due to this change, even though auto-commit is enabled, it doesn't show up in the consumer offsets topics. This breaks existing kafka monitoring tools.

One proposed solution is for Pinot to set a default group id when it was not explicitly configured by user (say, table name as the default group id).