ChatThreadOperation constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. ChatThreadOperation
4. ChatThreadOperation const constructor

ChatThreadOperation enum

1. Constructors
2. ChatThreadOperation
3. Values
4. UnKnown
5. Create
6. Update
7. Delete
8. Update_Msg
9. Properties
10. hashCode
11. index
12. runtimeType
13. Methods
14. noSuchMethod
15. toString
16. Operators
17. operator ==
18. Constants
19. values