Ensure that RR is not received for a while before running scorer on nil (#1653)

data.

Without the check, it was getting tripped by publisher not publishing
any data. Both conditions returned nil, but in one case, the receiver
report should have been received, but no movement in number of packets.

pkg/sfu/buffer/rtpstats.go

@@ -869,7 +869,7 @@ func (r *RTPStats) DeltaInfo(snapshotId uint32) *RTPDeltaInfo {
 	}
 	if packetsExpected == 0 {
 		if r.params.IsReceiverReportDriven {
-			// not received RTCP RR
+			// not received RTCP RR (OR) publisher is not producing any data
 			return nil
 		}