made updateDifficulty accept nullable difficulty

lib/services/providers/recipe_provider.dart

@@ -30,7 +30,7 @@ class RecipeProvider extends ChangeNotifier {
     }
   }
 
-  void updateDifficulty(Difficulty difficulty, {bool silent = false}) {
+  void updateDifficulty(Difficulty? difficulty, {bool silent = false}) {
     _recipe = _recipe.copyWith(difficulty: difficulty);
     if (!silent) {
       notifyListeners();