rtpsession: Always keep at least one NACK on early RTCP
We recently added code to remove outdate NACK to avoid using bandwidth for packet that have no chance of arriving on time. Though, this had a side effect, which is that it was to get an early RTCP packet with no feedback into it. This was pretty useless but also had a side effect, which is that the RTX RTT value would never be updated. So we we stared having late RTX request due to high RTT, we'd never manage to recover.
This fixes the regression by making sure we keep at least one NACK in this situation. This is really light on the bandwidth and allow for quick recover after the RTT have spiked higher then the jitterbuffer capacity.