维保记录列表，维保时间可能为null的bug修复

lift-business-service/src/main/java/cn/com/ty/lift/business/maintenance/service/MaintenanceRecordService.java

@@ -584,8 +584,12 @@ public class MaintenanceRecordService extends ServiceImpl<MaintenanceRecordMappe
         if (CollUtil.isNotEmpty(page.getRecords())) {
             List<MtRecordResponse> records = page.getRecords();
             for (MtRecordResponse mtRecord : records) {
-                String repairDuration = getRepairDuration(mtRecord.getRepairDiff());
-                mtRecord.setRepairDuration(repairDuration);
+                if (mtRecord.getRepairDiff() != null) {
+                    String repairDuration = getRepairDuration(mtRecord.getRepairDiff());
+                    mtRecord.setRepairDuration(repairDuration);
+                } else {
+                    mtRecord.setRepairDuration("");
+                }
             }
         }
         return page;