make TrashFile members late

lib/models/trash_file.dart

@@ -2,13 +2,13 @@ import 'package:photos/models/file.dart';
 
 class TrashFile extends File {
   // time when file was put in the trash for first time
-  int? createdAt;
+  late int createdAt;
 
   // for non-deleted trash items, updateAt is usually equal to the latest time
   // when the file was moved to trash
-  int? updateAt;
+  late int updateAt;
 
   // time after which will will be deleted from trash & user's storage usage
   // will go down
-  int? deleteBy;
+  late int deleteBy;
 }