NERtcChannelEventCallback constructor Null safety

1. nertc
2. nertc
3. NERtcChannelEventCallback
4. NERtcChannelEventCallback constructor

NERtcChannelEventCallback class

1. Constructors
2. NERtcChannelEventCallback
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onAudioHasHowling
9. onAudioRecording
10. onClientRoleChange
11. onConnectionStateChanged
12. onConnectionTypeChanged
13. onDisconnect
14. onError
15. onFirstAudioDataReceived
16. onFirstAudioFrameDecoded
17. onFirstVideoDataReceived
18. onFirstVideoFrameDecoded
19. onJoinChannel
20. onLeaveChannel
21. onLiveStreamState
22. onLocalAudioVolumeIndication
23. onLocalPublishFallbackToAudioOnly
24. onMediaRelayReceiveEvent
25. onMediaRelayStatesChange
26. onReceiveSEIMsg
27. onReconnectingStart
28. onReJoinChannel
29. onRemoteAudioVolumeIndication
30. onRemoteSubscribeFallbackToAudioOnly
31. onUserAudioMute
32. onUserAudioStart
33. onUserAudioStop
34. onUserJoined
35. onUserLeave
36. onUserSubStreamVideoStart
37. onUserSubStreamVideoStop
38. onUserVideoMute
39. onUserVideoProfileUpdate
40. onUserVideoStart
41. onUserVideoStop
42. onWarning
43. toString
44. Operators
45. operator ==