fix: Prevent clearing the category links list while merging

Author: AhmedElrakhawy

src/VirtoCommerce.CatalogCsvImportModule.Core/Model/CsvProduct.cs

@@ -433,6 +433,7 @@ public virtual void MergeFrom(CatalogProduct product)
         Height ??= product.Height;
         Length ??= product.Length;
         Width ??= product.Width;
+        Links ??= product.Links;
 
         MaxQuantity ??= product.MaxQuantity;
         MinQuantity ??= product.MinQuantity;