NERtcDeviceEventCallback constructor Null safety

1. nertc
2. nertc
3. NERtcDeviceEventCallback
4. NERtcDeviceEventCallback constructor

NERtcDeviceEventCallback class

1. Constructors
2. NERtcDeviceEventCallback
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onAudioDeviceChanged
9. onAudioDeviceStateChange
10. onVideoDeviceStageChange
11. toString
12. Operators
13. operator ==