Fix the inconsistency between snapshot_get_branches function in postgresql.storage and cassandra.storage

swh-storage.swh.storage.cassandra.storage and swh-storage.swh.storage.postgresql.storage implement the same interface. The "snapshot_get_branches function" is inconsistent between these two implementations.

postgresql storage could return None whereas the cassandra storage always returns a data structure. Fix this inconsistency.

This fix will cause unit tests to fail in swh-web.