fix: Handle null values in response categories and map properties correctly in GetDiscountCategoriesQueryHandler

src/BackOffice.BFF.Application/DiscountCategoryCQ/Queries/GetDiscountCategories/GetDiscountCategoriesQueryHandler.cs

@@ -25,7 +25,7 @@ public class GetDiscountCategoriesQueryHandler : IRequestHandler<GetDiscountCate
 
         return new GetDiscountCategoriesResponseDto
         {
-            Categories = response.Categories.Select(MapCategory).ToList()
+            Categories = response.Categories?.Select(MapCategory).ToList() ?? new List<DiscountCategoryTreeDto>()
         };
     }
 
@@ -36,14 +36,14 @@ public class GetDiscountCategoriesQueryHandler : IRequestHandler<GetDiscountCate
             Id = node.Id,
             Name = node.Name,
             Title = node.Title,
-            Description = string.IsNullOrWhiteSpace(node.Description) ? node.Description : string.Empty,
-            ImagePath =string.IsNullOrWhiteSpace(node.ImagePath)? node.ImagePath : string.Empty,
-            ParentCategoryId = node.ParentCategoryId!=null? node.ParentCategoryId.Value : null,
+            Description = !string.IsNullOrWhiteSpace(node.Description) ? node.Description : string.Empty,
+            ImagePath = !string.IsNullOrWhiteSpace(node.ImagePath) ? node.ImagePath : string.Empty,
+            ParentCategoryId = node.ParentCategoryId != 0 ? node.ParentCategoryId : null,
             SortOrder = node.SortOrder,
             IsActive = node.IsActive,
             ProductCount = node.ProductCount,
             Created = DateTime.UtcNow, // Proto doesn't have created field
-            ChildCategories = node.Children.Select(MapCategory).ToList()
+            ChildCategories = node.Children?.Select(MapCategory).ToList() ?? new List<DiscountCategoryTreeDto>()
         };
     }
 }