NIMClientType constructor Null safety

1. nim_core
2. nim_core
3. NIMClientType
4. NIMClientType const constructor

NIMClientType enum

1. Constructors
2. NIMClientType
3. Values
4. unknown
5. android
6. ios
7. windows
8. wp
9. web
10. rest
11. macos
12. Properties
13. hashCode
14. index
15. runtimeType
16. Methods
17. noSuchMethod
18. toString
19. Operators
20. operator ==
21. Constants
22. values