Don't bother with outer check for now

synapse/storage/client_ips.py

@@ -90,9 +90,6 @@ class ClientIpStore(background_updates.BackgroundUpdateStore):
             are (user_id, device_id) tuples. The values are also dicts, with
             keys giving the column names
         """
-        if not devices:
-            defer.returnValue({})
-
         res = yield self.runInteraction(
             "get_last_client_ip_by_device",
             self._get_last_client_ip_by_device_txn,