fix bugs

stmms-web/src/main/java/cn/com/qmth/stmms/api/controller/ExamStudentController.java

@@ -343,6 +343,9 @@ public class ExamStudentController extends BaseApiController {
             if (marker != null) {
                 markerNames.add(marker.getName());
             }
+            if (library.getHeaderId() == null) {
+                continue;
+            }
             User user = userService.findById(library.getHeaderId());
             if (user != null) {
                 headerNames.add(user.getName());