fix: query.map() and query.map_async() hanging with empty result set.

[chrisrossi]

Fixes#227.

[chrisrossi]

@cguardia Can you take a look when you get a chance?

[andrewsg]

@chrisrossi I'm seeing this in the Kokoro build failure:

==================================== ERRORS ====================================
_____________________ ERROR at teardown of test_filter_or ______________________
ds_client = <google.cloud.datastore.client.Client object at 0x7f6318196860>
to_delete = [<Key('SomeKind', 5723651571712000), project=precise-truck-742>, <Key('SomeKind', 5160701618290688), project=precise-truck-742>, <Key('SomeKind', 6286601525133312), project=precise-truck-742>]
deleted_keys = {<Key('SomeKind', 6376074283843584), project=precise-truck-742>, <Key('SomeKind', 5540445446733824), project=precise-t...d', 4796636970614784), project=precise-truck-742>, <Key('SomeKind', 6098997353644032), project=precise-truck-742>, ...}
@pytest.fixture
def with_ds_client(ds_client, to_delete, deleted_keys):
# Make sure we're leaving database as clean as we found it after each test
results = [
entity
for entity in all_entities(ds_client)
if entity.key not in deleted_keys
]
assert not results
yield ds_client
if to_delete:
ds_client.delete_multi(to_delete)
deleted_keys.update(to_delete)
not_deleted = [
entity
for entity in all_entities(ds_client)
if entity.key not in deleted_keys
]
> assert not not_deleted
E AssertionError: assert not [<Entity('SomeKind', 5112361526493184) {'foo': 1, 'bar': 'a'}>]
tests/system/conftest.py:76: AssertionError