fix bug

exam-work-api/src/main/java/cn/com/qmth/examcloud/service/examwork/service/ExamStudentService.java

@@ -420,6 +420,9 @@ public class ExamStudentService {
 		Exam exam = examRepo.findOne(examId);
 		if(exam.getCanStuDel()){
 			List<ExamStudent> examStudents = examStudentRepo.findByExamId(examId);
+			if(examStudents == null || examStudents.size() == 0){
+				return;
+			}
 			examStudentRepo.deleteInBatch(examStudents);
 		}else{
 			throw new RuntimeException("该考试已开始,不能删除");