allow nulls tracks to be passed in#4

cloudwebrtc merged 1 commit intoflutter-webrtc:mainflutter-webrtc/webrtc-interface:mainfrom

itsthetaste:support-null-tracks-in-sender-replaceTrack-and-setTrackitsthetaste/webrtc-interface:support-null-tracks-in-sender-replaceTrack-and-setTrackCopy head branch name to clipboard