participantCount property Null safety

1. sendbird_sdk
2. open_channel
3. OpenChannel
4. participantCount property

OpenChannel class

1. Constructors
2. OpenChannel
3. fromJson
4. fromJsonAndCached
5. Properties
6. channelType
7. channelUrl
8. coverUrl
9. createdAt
10. creator
11. customType
12. data
13. dirty
14. entered
15. fromCache
16. hashCode
17. isEphemeral
18. isFrozen
19. key
20. name
21. operators
22. participantCount
23. primaryKey
24. runtimeType
25. Methods
26. copyWith
27. deleteChannel
28. isOperator
29. noSuchMethod
30. toJson
31. toString
32. updateChannel
33. Operators
34. operator ==
35. Static methods
36. createChannel
37. getChannel
38. refresh