BigQuery dataset.list_tables() fails when the dataset is empty

Good day.

Getting this exception when the BigQuery dataset contains no tables:

``` python
  File "/Library/Python/2.7/site-packages/gcloud/bigquery/dataset.py", line 507, in list_tables
    for resource in resp['tables']]
KeyError: 'tables'
```

Mac OS X El Capitan. gcloud 0.8.0
