userId property Null safety

1. sendbird_sdk
2. state
3. SendbirdState
4. userId property

SendbirdState class

1. Constructors
2. SendbirdState
3. Properties
4. apiHost
5. appId
6. appInfo
7. appVersion
8. connected
9. connecting
10. currentUser
11. hasActiveUser
12. hashCode
13. lastConnectedAt
14. maxUnreadCountOnSuperGroup
15. pushToken
16. pushTokenType
17. reconnectConfig
18. reconnecting
19. reconnectTask
20. runtimeType
21. sessionKey
22. token
23. unreadCountInfo
24. uploadSizeLimit
25. userId
26. wsHost
27. Methods
28. cleanUp
29. noSuchMethod
30. resetReconnectTask
31. toString
32. Operators
33. operator ==