skip left/invited rooms with no updates in /sync

Before this change we were just returning an empty object for left or
invited rooms that don't have any updates. This is valid coloredding to
the spec, but it's a nicer to debug if they are omitted it and results in
a little less network traffic. For joined rooms, we are already skipping
empty updates.

With filtering support, it's much more common to have sync responses where
many rooms are empty, because all of the state/timeline events may be
filtered out.

src/api/client_server/sync.rs

@@ -460,16 +460,16 @@ async fn sync_helper(
 			}
 		};
 
-		left_rooms.insert(
-			room_id.into_owned(),
-			LeftRoom {
-				account_data: RoomAccountData {
-					events: Vec::new(),
-				},
-				timeline,
-				state,
+		let left_room = LeftRoom {
+			account_data: RoomAccountData {
+				events: Vec::new(),
 			},
-		);
+			timeline,
+			state,
+		};
+		if !left_room.is_empty() {
+			left_rooms.insert(room_id.into_owned(), left_room);
+		}
 	}
 
 	let mut invited_rooms = BTreeMap::new();
@@ -519,14 +519,14 @@ async fn sync_helper(
 			continue;
 		}
 
-		invited_rooms.insert(
-			room_id.into_owned(),
-			InvitedRoom {
-				invite_state: InviteState {
-					events: invite_state_events,
-				},
+		let invited_room = InvitedRoom {
+			invite_state: InviteState {
+				events: invite_state_events,
 			},
-		);
+		};
+		if !invited_room.is_empty() {
+			invited_rooms.insert(room_id.into_owned(), invited_room);
+		}
 	}
 
 	for user_id in left_encrypted_users {