If AutoAck pull returns empty then exception raised on context exit

[quom]

If the subscription pull returns 0 messages in the enter method in the AutoAck class then on exit the acknowledge call raises an exception on the HTTP call: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/pubsub-0.22.0/pubsub/google/cloud/pubsub/subscription.py#L512

Currently the way a deal with this is to wrap the context manager in try/except or override exit to check for an empty list. The former is messy and the latter can just be fixed at source.

[tseaver]

@quom Thanks for the report! It looks as though you are working on a fix. Can you also add a test which fails without your patch?

[quom]

@tseaver There you go :)

[tseaver]

@quom Looking good. PR forthcoming?