BigQuery client.list_datasets() fails when the project has no datasets

[dhermes]

Found while doing clean-up of a project after reproducing #1419

Reproduce:

In [11]: client.list_datasets()
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-11-fed4650eba20> in <module>()
----> 1 client.list_datasets()

gcloud-python/gcloud/bigquery/client.pyc in list_datasets(self, include_all, max_results, page_token)
     89                                            query_params=params)
     90         datasets = [Dataset.from_api_repr(resource, self)
---> 91                     for resource in resp['datasets']]
     92         return datasets, resp.get('nextPageToken')
     93 

KeyError: 'datasets'``

[dhermes]

Full payload of response

{
  "etag": "\"AMpOK...\"", 
  "kind": "bigquery#datasetList"
}