NERtcStatsEventCallback constructor Null safety

1. nertc
2. nertc
3. NERtcStatsEventCallback
4. NERtcStatsEventCallback constructor

NERtcStatsEventCallback class

1. Constructors
2. NERtcStatsEventCallback
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onLocalAudioStats
9. onLocalVideoStats
10. onNetworkQuality
11. onRemoteAudioStats
12. onRemoteVideoStats
13. onRtcStats
14. toString
15. Operators
16. operator ==