toJson method Null safety

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

CheckoutBillingAddress class

1. Constructors
2. CheckoutBillingAddress
3. fromJson
4. Properties
5. addressLine1
6. addressLine2
7. city
8. country
9. hashCode
10. runtimeType
11. state
12. Methods
13. noSuchMethod
14. toJson
15. toString
16. Operators
17. operator ==