restServer property Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. ChatOptions
4. restServer property

ChatOptions class

1. Constructors
2. ChatOptions
3. Properties
4. acceptInvitationAlways
5. appKey
6. areaCode
7. autoAcceptGroupInvitation
8. autoLogin
9. debugModel
10. deleteMessagesAsExitChatRoom
11. deleteMessagesAsExitGroup
12. dnsUrl
13. enableDNSConfig
14. hashCode
15. imPort
16. imServer
17. isAutoDownloadThumbnail
18. isChatRoomOwnerLeaveAllowed
19. requireAck
20. requireDeliveryAck
21. restServer
22. runtimeType
23. serverTransfer
24. sortMessageByServerTime
25. usingHttpsOnly
26. Methods
27. enableAPNs
28. enableFCM
29. enableHWPush
30. enableMeiZuPush
31. enableMiPush
32. enableOppoPush
33. enableVivoPush
34. noSuchMethod
35. toString
36. Operators
37. operator ==