birthday property Null safety

1. wings_data_model
2. wings_data_model
3. UserBuilder
4. birthday property

UserBuilder class

1. Constructors
2. UserBuilder
3. Properties
4. avatar
5. birthday
6. country
7. email
8. fcmToken
9. fcmTokenDateTime
10. firebaseUid
11. firstName
12. gender
13. hashCode
14. interests
15. isCrashlyticsAllowed
16. isMedicalProfileMiningAllowed
17. isOpenForThirdPartyContacts
18. language
19. lastName
20. mobilePhone
21. runtimeType
22. userId
23. zipCode
24. Methods
25. noSuchMethod
26. replace
27. toString
28. update
29. Operators
30. operator ==