toJson method Null safety

1. baguette
2. CheckoutPhone
3. CheckoutPhone
4. toJson method

CheckoutPhone class

1. Constructors
2. CheckoutPhone
3. fromJson
4. Properties
5. countryCode
6. hashCode
7. number
8. runtimeType
9. Methods
10. noSuchMethod
11. toJson
12. toString
13. Operators
14. operator ==