fix : can't push state bugs

src/main/java/nju/seec/helper/entity/QuizStudentAnswer.java

@@ -49,7 +49,7 @@ public class QuizStudentAnswer {
     private LocalDateTime modifiedAt;
 
     @Basic
-    @Column(name = "studentAnswer",nullable = false)
+    @Column(name = "student_answer",nullable = true)
     private String studentAnswer;
 
     @Basic