video property Null safety

1. volc_engine_rtc
2. bytertc_video_defines
3. LiveTranscoding
4. video property

LiveTranscoding class

1. Constructors
2. LiveTranscoding
3. Properties
4. audio
5. authInfo
6. hashCode
7. layout
8. mixType
9. roomId
10. runtimeType
11. uid
12. url
13. video
14. Methods
15. noSuchMethod
16. toString
17. Operators
18. operator ==