easy constant Null safety

1. wings_data_model
2. wings_data_model
3. ContentElementDifficulty
4. easy constant

ContentElementDifficulty class

1. Properties
2. hashCode
3. name
4. runtimeType
5. Methods
6. noSuchMethod
7. toString
8. Operators
9. operator ==
10. Static properties
11. values
12. Static methods
13. fromDatabaseFormat
14. toDatabaseFormat
15. valueOf
16. Constants
17. easy
18. hard
19. normal