Allow empty list [] in push to /v1/repositories/<path>/

Multiple docker pushes to the same repository path will end up sending [] to /v1/repositories/<path>/.

Currently we api_error 400 in this case, which the docker client doesn't like.

This is already handled properly in update_index_images, so we might as well allow it.
Merge pull request #57 from avaer/master

Allow empty list [] in push to /v1/repositories/<path>/