onlineState property Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. ChatMessage
4. onlineState property

ChatMessage class

1. Constructors
2. createCmdSendMessage
3. createCustomSendMessage
4. createFileSendMessage
5. createImageSendMessage
6. createLocationSendMessage
7. createReceiveMessage
8. createSendMessage
9. createTxtSendMessage
10. createVideoSendMessage
11. createVoiceSendMessage
12. Properties
13. attributes
14. body
15. chatType
16. conversationId
17. direction
18. from
19. hasDeliverAck
20. hashCode
21. hasRead
22. hasReadAck
23. isChatThreadMessage
24. localTime
25. msgId
26. needGroupAck
27. onlineState
28. runtimeType
29. serverTime
30. status
31. to
32. Methods
33. chatThread
34. dispose
35. groupAckCount
36. noSuchMethod
37. reactionList
38. setMessageStatusCallBack
39. toString
40. Operators
41. operator ==