fontSize property Null safety

1. flutter_json_widgets
2. text_style
3. DisplaySmall
4. fontSize property

DisplaySmall class

1. Constructors
2. DisplaySmall
3. fromJson
4. Properties
5. background
6. backgroundColor
7. color
8. copyWith
9. debugLabel
10. decoration
11. decorationColor
12. decorationStyle
13. decorationThickness
14. fontFamily
15. fontFamilyFallback
16. fontFeatures
17. fontSize
18. fontStyle
19. fontVariations
20. fontWeight
21. foreground
22. hashCode
23. height
24. inherit
25. leadingDistribution
26. letterSpacing
27. locale
28. overflow
29. package
30. runtimeType
31. shadows
32. textBaseline
33. wordSpacing
34. Methods
35. map
36. mapOrNull
37. maybeMap
38. maybeWhen
39. noSuchMethod
40. toJson
41. toString
42. when
43. whenOrNull
44. Operators
45. operator ==