NIMMessageDirection constructor Null safety

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

NIMMessageDirection enum

1. Constructors
2. NIMMessageDirection
3. Values
4. outgoing
5. received
6. Properties
7. hashCode
8. index
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==
15. Constants
16. values