fromAccount property Null safety

1. nim_core
2. nim_core
3. NIMMessage
4. fromAccount property

NIMMessage class

1. Constructors
2. NIMMessage
3. audioEmptyMessage
4. customEmptyMessage
5. emptyMessage
6. fileEmptyMessage
7. fromMap
8. imageEmptyMessage
9. locationEmptyMessage
10. textEmptyMessage
11. tipEmptyMessage
12. videoEmptyMessage
13. Properties
14. ackCount
15. antiSpamOption
16. attachmentStatus
17. callbackExtension
18. clientAntiSpam
19. config
20. content
21. env
22. fromAccount
23. fromNickname
24. hashCode
25. hasSendAck
26. isChecked
27. isDeleted
28. isInBlackList
29. isRemoteRead
30. localExtension
31. memberPushOption
32. messageAck
33. messageAttachment
34. messageDirection
35. messageId
36. messageSubType
37. messageThreadOption
38. messageType
39. pushContent
40. pushPayload
41. quickCommentUpdateTime
42. remoteExtension
43. runtimeType
44. senderClientType
45. serverId
46. sessionId
47. sessionType
48. sessionUpdate
49. status
50. timestamp
51. unAckCount
52. uuid
53. yidunAntiCheating
54. Methods
55. noSuchMethod
56. toMap
57. toString
58. Operators
59. operator ==