NEMessageService constructor Null safety

1. yunxin_roomkit
2. roomkit
3. NEMessageService
4. NEMessageService constructor

NEMessageService class

1. Constructors
2. NEMessageService
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. addMessageCallback
8. noSuchMethod
9. removeMessageCallback
10. sendPassThroughMessage
11. toString
12. Operators
13. operator ==