ChannelEventCategory constructor Null safety

1. sendbird_sdk
2. enums
3. ChannelEventCategory
4. ChannelEventCategory const constructor

ChannelEventCategory enum

1. Constructors
2. ChannelEventCategory
3. Values
4. none
5. join
6. leave
7. updateOperators
8. invite
9. declineInvite
10. enter
11. exit
12. mute
13. unmute
14. ban
15. unban
16. frozen
17. unfrozen
18. typingStart
19. typingEnd
20. propChanged
21. deleted
22. metaDataChanged
23. metaCounterChanged
24. hidden
25. unhidden
26. pinnedMessage
27. Properties
28. hashCode
29. index
30. runtimeType
31. Methods
32. noSuchMethod
33. toString
34. Operators
35. operator ==
36. Constants
37. values