Write_Operation constructor Null safety

1. spine_client
2. write.pb
3. Write_Operation
4. Write_Operation const constructor

Write_Operation enum

1. Constructors
2. Write_Operation
3. Values
4. update
5. delete
6. transform
7. notSet
8. Properties
9. hashCode
10. index
11. runtimeType
12. Methods
13. noSuchMethod
14. toString
15. Operators
16. operator ==
17. Constants
18. values