serviceName property Null safety

1. nim_core
2. nim_core
3. SystemMessageServicePlatform
4. serviceName property

SystemMessageServicePlatform class

1. Constructors
2. SystemMessageServicePlatform
3. Properties
4. hashCode
5. onCustomNotification
6. onReceiveSystemMessage
7. onUnreadCountChange
8. runtimeType
9. serviceName
10. Methods
11. clearSystemMessages
12. clearSystemMessagesByType
13. deleteSystemMessage
14. handleMethodCall
15. invokeMethod
16. noSuchMethod
17. onEvent
18. querySystemMessageByTypeAndroid
19. querySystemMessageByTypeIOSAndDesktop
20. querySystemMessagesAndroid
21. querySystemMessagesIOSAndDesktop
22. querySystemMessageUnread
23. querySystemMessageUnreadCount
24. querySystemMessageUnreadCountByType
25. resetSystemMessageUnreadCount
26. resetSystemMessageUnreadCountByType
27. sendCustomNotification
28. setSystemMessageRead
29. setSystemMessageStatus
30. toString
31. Operators
32. operator ==
33. Static properties
34. instance