fix: 空SKU编码时改为自动生成避免冲突

src/Api/TakeoutSaaS.TenantApi/Services/ProductSkuSaveService.cs

@@ -147,7 +147,9 @@ public sealed class ProductSkuSaveService(TakeoutAppDbContext dbContext)
                 continue;
             }
 
-            var generatedCode = normalizedSkuCode ?? GenerateUniqueSkuCode(productId, currentProductSkuCodes);
+            var generatedCode = !string.IsNullOrWhiteSpace(normalizedSkuCode)
+                ? normalizedSkuCode
+                : GenerateUniqueSkuCode(productId, currentProductSkuCodes);
             if (!plannedSkuCodes.Add(generatedCode))
             {
                 throw new BusinessException(ErrorCodes.BadRequest, $"SKU 编码冲突: {generatedCode}");