lastFour property Null safety

1. baguette
2. CheckoutTokenResponse
3. CheckoutTokenResponse
4. lastFour property

CheckoutTokenResponse class

1. Constructors
2. CheckoutTokenResponse
3. fromJson
4. Properties
5. billingAddress
6. bin
7. cardCategory
8. cardType
9. expiresOn
10. expiryMonth
11. expiryYear
12. hashCode
13. issuer
14. issuerCountry
15. lastFour
16. name
17. phone
18. productID
19. productType
20. runtimeType
21. scheme
22. token
23. type
24. Methods
25. noSuchMethod
26. toJson
27. toString
28. Operators
29. operator ==
30. Constants
31. APPLE_PAY
32. CARD
33. GOOGLE_PAY