null Track in RtpSender

[aboba]

In WebRTC 1.0, replaceTrack(null) is permitted, and also, an RtpSender can be constructed with a null track. So ORTC needs to be compatible with this.

[fippo]

if no track is provided a kind is probably required -- that probably implies a constructor that takes a trackOrKind similar to the 1.0 createTransceiver.