Should Sender replaceTrack and setTrack support receiving a null track

[itsthetaste]

I'm working on a PR for flutter-webrtc, trying to add the ability to send null into replaceTrack and setTrack, but this interface won't allow that.
Seems a simple fix, something like

Future replaceTrack(MediaStreamTrack? track);

Future setTrack(MediaStreamTrack? track, {bool takeOwnership = true});

I'll try and see if it works and submit a PR if ok

[cloudwebrtc]

@itsthetaste I think it makes sense and I'd be happy to accept a PR to fix this issue.

[itsthetaste]

I've made this #4