url property Null safety

1. sendbird_sdk
2. file_info
3. FileInfo
4. url property

FileInfo class

1. Constructors
2. FileInfo
3. fromBytes
4. fromData
5. fromJson
6. fromUrl
7. Properties
8. file
9. fileBytes
10. fileSize
11. hasBinary
12. hashCode
13. hasSource
14. mimeType
15. name
16. runtimeType
17. url
18. Methods
19. noSuchMethod
20. toJson
21. toString
22. Operators
23. operator ==