Default "" consumer group tracks committed offsets, but is otherwise not a real group

To reproduce:

• Use the default config for `group.id` of "" (the empty string)
• Use the default config for `enable.auto.commit` of `true`
• Use manually assigned partitions with `assign`

Actual (unexpected) behaviour:

Consumer offsets are stored for the "" group. Example:

~ $ /opt/kafka/kafka_2.11-0.11.0.2/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group ""
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).

Consumer group '' has no active members.

TOPIC PARTITION CURRENT-OFFSET LOG-END-OFFSET LAG CONSUMER-ID HOST CLIENT-ID
my_topic 54 7859593 7865082 5489 - - -
my_topic 5 14252813 14266419 13606 - - -
my_topic 39 19099099 19122441 23342 - - -
my_topic 43 16434573 16449180 14607 - - -.

....

However, the "" is not a real group. It doesn't show up with:

~ $ /opt/kafka/kafka_2.11-0.11.0.2/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --list

You also can't do dynamic partition assignment with it - if you try to `subscribe` when using the default "" group ID, you get:

AbstractCoordinator: Attempt to join group  failed due to fatal error: The configured groupId is invalid

Better behaviours:

(any of these would be preferable, in my opinion)

• Don't commit offsets with the "" group, and log a warning telling the user that `enable.auto.commit = true` is meaningless in this situation. This is what I would have expected.
• Don't have a default `group.id`. Some of my reading indicates that the new consumer basically needs a `group.id` to function. If so, force users to choose a group ID so that they're more aware of what will happen.
• Have a default `group.id` of `default`, and make it a real consumer group. That is, it shows up in lists of groups, it has dynamic partitioning, etc.

As a user, when I don't set `group.id` I expect that I'm not using consumer groups. This is confirmed to me by listing the consumer groups on the broker and not seeing anything. Therefore, I expect that there will be no offset tracking in Kafka.

In my specific application, I was wanting `auto.offset.reset` to kick in so that a failed consumer would start at the `latest` offset. However, it started at this unexpectedly stored offset instead.