Don't query for to-device messages without a device

synapse/handlers/sync.py

@@ -1335,7 +1335,7 @@ async def _generate_sync_entry_for_to_device(
         if sync_result_builder.since_token is not None:
             since_stream_id = int(sync_result_builder.since_token.to_device_key)
 
-        if since_stream_id != int(now_token.to_device_key):
+        if device_id is not None and since_stream_id != int(now_token.to_device_key):
             messages, stream_id = await self.store.get_messages_for_device(
                 user_id, device_id, since_stream_id, now_token.to_device_key
             )