raise unexpected exception while property value is []

[lucemia]

the following sample code

entity = Entity(dataset, 'test')
entity['a'] = []
entity.save()

will raise exception

File "/usr/local/lib/python2.7/dist-packages/gcloud-0.3.0-py2.7.egg/gcloud/datastore/entity.py", line 244, in save
    exclude_from_indexes=self.exclude_from_indexes())
  File "/usr/local/lib/python2.7/dist-packages/gcloud-0.3.0-py2.7.egg/gcloud/datastore/connection.py", line 429, in save_entity
    result = self.commit(dataset_id, mutation)
  File "/usr/local/lib/python2.7/dist-packages/gcloud-0.3.0-py2.7.egg/gcloud/datastore/connection.py", line 329, in commit
    datastore_pb.CommitResponse)
  File "/usr/local/lib/python2.7/dist-packages/gcloud-0.3.0-py2.7.egg/gcloud/datastore/connection.py", line 81, in _rpc
    data=request_pb.SerializeToString())
  File "/usr/local/lib/python2.7/dist-packages/google/protobuf/internal/python_message.py", line 813, in SerializeToString
    self.DESCRIPTOR.full_name, ','.join(self.FindInitializationErrors())))
EncodeError: Message api.services.datastore.CommitRequest is missing required fields: mutation.upsert[0].property[0].value

It (protobuf) makes no distinction between an empty list and a null list.

http://stackoverflow.com/questions/2378666/protobuf-net-serializing-an-empty-list
it looks like a property with empty list should be skip entirely

[tseaver]

https://github.com/tseaver/gcloud-python/commit/876d7fe75c9c9277dfcd05d00160e75f53658d82 is an alternate approach: it marshalls an empty list to-from a custom sub-entity. AFAICT the sub-entity approach (which might also be useful for making 'None' an explicit value) only works if the "empty list" field is excluded from indexing.

[dhermes]

I actually dove in and tried to make this work at the proto level and agree with @bryanyang0528 that it should be skipped entirely.

[dhermes]

Fixed by #512