fix: getParkingDiscounts 断言写反导致功能完全不可用

- 原断言 isEmptyOrNull(orders)：有洗车记录的用户反而报'无停车场洗车记录'，
  无记录的用户走到 orders.get(0) 抛 IndexOutOfBoundsException
- 修正为 !isEmptyOrNull(orders)，同时移除恒真的 sum()>=0 冗余条件

Co-Authored-By: Claude <noreply@anthropic.com>

car-wash-service/src/main/java/com/kym/service/impl/WashOrderServiceImpl.java

@@ -315,8 +315,8 @@ public class WashOrderServiceImpl extends MyBaseServiceImpl<WashOrderMapper, Was
                 .orderByDesc(WashOrder::getId)
                 .list();
 
-        CommUtil.asserts(CommUtil.isEmptyOrNull(orders) && (orders.stream().mapToInt(WashOrder::getAmount).sum() >= 0),
-                "抱歉：无停车场洗车记录");
+        // 原断言写反：orders 非空时抛异常、为空时走到 orders.get(0) NPE，此处修正
+        CommUtil.asserts(!CommUtil.isEmptyOrNull(orders), "抱歉：无停车场洗车记录");
 
         return washStationService.lambdaQuery().eq(WashStation::getStationId, orders.get(0).getStationId()).one().getParkingQrCode();
     }