RemoteGraphClient returns a list with an empty element when there is no answers

RemoteGraphClient returns a list with an empty string when there is no results. It should return an empty list instead.

Example:

>>> from swh.graph.http_client import RemoteGraphClient
>>> client = RemoteGraphClient("http://granet.internal.softwareheritage.org:5009/graph")
>>> list(client.neighbors("swh:1:dir:db40b68089ab62e6e8fc885d70d40345793dd9da", direction="backward", max_edges=1))
['']