size property Null safety

1. sendbird_sdk
2. file_message
3. FileMessage
4. size property

FileMessage class

1. Constructors
2. FileMessage
3. fromJson
4. fromParams
5. Properties
6. channelType
7. channelUrl
8. createdAt
9. customType
10. data
11. errorCode
12. extendedMessage
13. forceUpdateLastMessage
14. hashCode
15. isOperatorMessage
16. isPinnedMessage
17. isSilent
18. localFile
19. mentionedUserIds
20. mentionedUsers
21. mentionType
22. message
23. messageId
24. messageSurvivalSeconds
25. metaArrays
26. name
27. ogMetaData
28. parentMessage
29. parentMessageId
30. parentMessageText
31. reactions
32. replyToChannel
33. requestedMentionUserIds
34. requestId
35. requireAuth
36. runtimeType
37. scheduledInfo
38. secureUrl
39. sender
40. sendingStatus
41. size
42. threadInfo
43. thumbnails
44. type
45. updatedAt
46. url
47. Methods
48. applyReactionEvent
49. applyThreadInfoUpdateEvent
50. getMetaArrays
51. getThreadedMessagesByTimestamp
52. isResendable
53. noSuchMethod
54. toJson
55. toString
56. Operators
57. operator ==