PhotoRepository constructor Null safety

1. next_photo
2. features
3. PhotoRepository
4. PhotoRepository constructor

PhotoRepository class

1. Constructors
2. PhotoRepository
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. getAlbumPhotos
8. getAllPhotos
9. getPhoto
10. noSuchMethod
11. setPhotoLike
12. toString
13. Operators
14. operator ==