made difficulty nullable

lib/models/recipe.dart

@@ -6,7 +6,7 @@ class Recipe {
     this.id = -1,
     this.title = '',
     this.description = '',
-    this.difficulty = Difficulty.notSelected,
+    this.difficulty,
     required this.datePublished,
     this.prepTime = const Duration(),
     this.cookTime = const Duration(),
@@ -22,7 +22,7 @@ class Recipe {
   final Cuisine? cuisine;
   final DateTime datePublished;
   final String description;
-  final Difficulty difficulty;
+  final Difficulty? difficulty;
   final int id;
   final List<IngredientListEntry> ingredients;
   final List<String> keywords;