ChatManagerListener constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. ChatManagerListener
4. ChatManagerListener constructor

ChatManagerListener class

1. Constructors
2. ChatManagerListener
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onCmdMessagesReceived
9. onConversationRead
10. onConversationsUpdate
11. onGroupMessageRead
12. onMessageReactionDidChange
13. onMessagesDelivered
14. onMessagesRead
15. onMessagesRecalled
16. onMessagesReceived
17. onReadAckForGroupMessageUpdated
18. toString
19. Operators
20. operator ==