Fix UT `Should_Add_Order_Candidate_Discount_Preview`

plugins/Promotions/test/EasyAbp.EShop.Plugins.Promotions.Application.Tests/PromotionTypes/MinQuantityOrderDiscountTests.cs

@@ -40,6 +40,9 @@ public class MinQuantityOrderDiscountTests : PromotionsApplicationTestBase
             ProductSkus = new List<ProductSkuEto>
             {
                 new()
+                {
+                    Currency = "USD"
+                }
             }
         };