考生作答时，设置并保存上次活动时间，可弥补缺失心跳接口调用情况

examcloud-core-oe-student-service/src/main/java/cn/com/qmth/examcloud/core/oe/student/service/impl/ExamRecordDataServiceImpl.java

@@ -360,6 +360,8 @@ public class ExamRecordDataServiceImpl implements ExamRecordDataService {
         long startTime = System.currentTimeMillis();
         ExamRecordQuestions examRecordQuestions = examRecordQuestionsService.getExamRecordQuestions(req.getExamRecordDataId());
 
+        // todo NPE
+
         // 考生作答记录明细
         List<ExamQuestion> examQuestions = examRecordQuestions.getExamQuestions();