Use if `is not None`

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/storage/client_ips.py

@@ -393,7 +393,7 @@ class ClientIpStore(background_updates.BackgroundUpdateStore):
         """
 
         keyvalues = {"user_id": user_id}
-        if device_id:
+        if device_id is not None:
             keyvalues["device_id"] = device_id
 
         res = yield self._simple_select_list(