FilterByDifficulty testing

Francesco.Recipes.World/Repositories/Recipe/IRecipeRepository.cs

@@ -14,7 +14,7 @@
 
         Task<IEnumerable<Recipe>> GetRecipesByNameOrIngredientAsync(string name, string ingredient);
 
-        Task<IEnumerable<Recipe>> GetRecipesByDifficultyAsync(Difficulty difficulty);
+        Task<IReadOnlyCollection<Recipe>> GetRecipesByDifficultyAsync(Difficulty? difficulty);
 
         Task<Unit> AddUnitToRecipeAsync(string name, string symbol);
 

Francesco.Recipes.World/Repositories/Recipe/RecipeRepository.cs

@@ -160,7 +160,7 @@
             return await query.ToListAsync();
         }
 
-        public async Task<IEnumerable<Recipe>> GetRecipesByDifficultyAsync(Difficulty difficulty)
+        public async Task<IReadOnlyCollection<Recipe>> GetRecipesByDifficultyAsync(Difficulty? difficulty)
         {
           return await _context.Recipes
               .Where(r => r.Difficulty == difficulty)