fix: 首页用户统计 groupingBy 遇到 null stationId 抛 NPE，改为映射到"未分配"

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -318,7 +318,7 @@ public class UserServiceImpl extends MPJBaseServiceImpl<UserMapper, User> implem
             wrapper.in(User::getStationId, Arrays.asList(stationId));
         }
         // 按照站点分组统计订单数量
-        return list(wrapper).stream().collect(Collectors.groupingBy(User::getStationId, Collectors.summingInt(o -> 1)));
+        return list(wrapper).stream().collect(Collectors.groupingBy(u -> u.getStationId() != null ? u.getStationId() : "未分配", Collectors.summingInt(o -> 1)));
     }
 
     /**
@@ -334,7 +334,7 @@ public class UserServiceImpl extends MPJBaseServiceImpl<UserMapper, User> implem
         LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
         wrapper.ge(User::getCreateTime, startTime);
         wrapper.lt(User::getCreateTime, endTime);
-        return list(wrapper).stream().collect(Collectors.groupingBy(User::getStationId, Collectors.summingInt(o -> 1)));
+        return list(wrapper).stream().collect(Collectors.groupingBy(u -> u.getStationId() != null ? u.getStationId() : "未分配", Collectors.summingInt(o -> 1)));
     }