ChatContactManagerListener constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. ChatContactManagerListener
4. ChatContactManagerListener constructor

ChatContactManagerListener class

1. Constructors
2. ChatContactManagerListener
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onContactAdded
9. onContactDeleted
10. onContactInvited
11. onFriendRequestAccepted
12. onFriendRequestDeclined
13. toString
14. Operators
15. operator ==