Bigquery API: query_response_to_dict raises exception for nullable values.

[alex-kowalczyk]

The following exception is raised by gcloud.aio.bigquery.query_response_to_dict for a nullable integer field with:

• schema {'name': 'fieldName', 'type': 'INTEGER', 'mode': 'NULLABLE'}
• received value {'v': None}:

    items = query_response_to_dict(query_resp)
  File "gcloud/aio/bigquery/utils.py", line 142, in query_response_to_dict
    return [
  File "gcloud/aio/bigquery/utils.py", line 143, in <listcomp>
    {k['name']: parse(k, v) for k, v in zip(fields, row)}
  File "gcloud/aio/bigquery/utils.py", line 143, in <dictcomp>
    {k['name']: parse(k, v) for k, v in zip(fields, row)}
  File "gcloud/aio/bigquery/utils.py", line 126, in parse
    return convert(flatten(value))
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

I think this check in parse function (107-108) is incorrect

    if field['mode'] == 'NULLABLE' and value is None:
        return value

It should be something like ... and flatten(value) is None: