syncapi/requestpool: fix initial sync logic error in appendAccountData() (#1594)

* requestpool: fix initial sync logic error in appendAccountData()

In initial sync, req.since is no longer nil, but instead,
req.since.PDUPosition() and req.since.EDUPosition() returns 0.

This ensures forgotten rooms do not come back as zombies.

* syncapi/requestpool: reintroduce req.since == nil check

syncapi/sync/requestpool.go

@@ -328,7 +328,7 @@ func (rp *RequestPool) appendAccountData(
 	// data keys were set between two message. This isn't a huge issue since the
 	// duplicate data doesn't represent a huge quantity of data, but an optimisation
 	// here would be making sure each data is sent only once to the client.
-	if req.since == nil {
+	if req.since == nil || (req.since.PDUPosition() == 0 && req.since.EDUPosition() == 0) {
 		// If this is the initial sync, we don't need to check if a data has
 		// already been sent. Instead, we send the whole batch.
 		dataReq := &userapi.QueryAccountDataRequest{