Fix TypeError in synapse.notifier (#7880)

Fixes #7774

changelog.d/7880.bugfix

@@ -0,0 +1 @@
+Fix "TypeError in `synapse.notifier`" exceptions.

synapse/handlers/device.py

@@ -421,6 +421,10 @@ class DeviceHandler(DeviceWorkerHandler):
         """Notify that a user's device(s) has changed. Pokes the notifier, and
         remote servers if the user is local.
         """
+        if not device_ids:
+            # No changes to notify about, so this is a no-op.
+            return
+
         users_who_share_room = await self.store.get_users_who_share_room_with_user(
             user_id
         )
@@ -436,6 +440,10 @@ class DeviceHandler(DeviceWorkerHandler):
             user_id, device_ids, list(hosts)
         )
 
+        if not position:
+            # This should only happen if there are no updates, so we bail.
+            return
+
         for device_id in device_ids:
             logger.debug(
                 "Notifying about update %r/%r, ID: %r", user_id, device_id, position