Remove empty fields from /sync response (#2755)

First attempt at removing empty fields from `/sync` responses. Needs
https://github.com/matrix-org/sytest/pull/1298 to keep Sytest happy.

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

syncapi/streams/stream_accountdata.go

@@ -90,9 +90,9 @@ func (p *AccountDataStreamProvider) IncrementalSync(
 				}
 			} else {
 				if roomData, ok := dataRes.RoomAccountData[roomID][dataType]; ok {
-					joinData := *types.NewJoinResponse()
-					if existing, ok := req.Response.Rooms.Join[roomID]; ok {
-						joinData = existing
+					joinData, ok := req.Response.Rooms.Join[roomID]
+					if !ok {
+						joinData = types.NewJoinResponse()
 					}
 					joinData.AccountData.Events = append(
 						joinData.AccountData.Events,