Role constructor Null safety

1. sendbird_sdk
2. enums
3. Role
4. Role const constructor

Role enum

1. Constructors
2. Role
3. Values
4. none
5. chat_operator
6. Properties
7. hashCode
8. index
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==
15. Constants
16. values