handler/sync: Handle batches without events

Check if the batch contains events before trying to access them.

Fixes matrix-org#4842

synapse/handlers/sync.py

@@ -770,7 +770,7 @@ def compute_state_delta(self, room_id, batch, sync_config, since_token, now_toke
                     current=current_state_ids,
                     lazy_load_members=lazy_load_members,
                 )
-            elif batch.limited:
+            elif batch.limited and batch.events:
                 state_at_timeline_start = yield self.store.get_state_ids_for_event(
                     batch.events[0].event_id, state_filter=state_filter,
                 )