courses property Null safety

1. univalle_app
2. subjects
3. SubjectListState
4. courses property

SubjectListState class

1. Constructors
2. SubjectListState
3. Properties
4. courses
5. errorMessage
6. hashCode
7. props
8. runtimeType
9. status
10. stringify
11. Methods
12. copyWith
13. noSuchMethod
14. toString
15. Operators
16. operator ==