Pubsub.list_topics fails when there are no topics

[dhermes]

Offending line. Reproduce via:

client = pubsub.Client()
>>> client.list_topics()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gcloud/pubsub/client.py", line 80, in list_topics
    for resource in resp['topics']]
KeyError: 'topics'

@tseaver ISTM we should locate all instances where we assume a key is present and just protect against this. The time between releases behooves us to be "protective" of users. (I realize that we've usually done it this way based on documented outputs.)

[dhermes]

Found while reproducing #1438