fix: json格式错误导致查看原卷报错

src/features/examwork/StudentExamDetail/PreviewPaperDialog.vue

@@ -139,12 +139,12 @@ export default {
         const k = `${question.mainNumber}-${question.subNumber}`;
         if (question.subIndex !== null) {
           studentAnsMap[`${k}-${question.subIndex}`] = {
-            answer: JSON.parse(question.answer),
+            answer: question.answer ? JSON.parse(question.answer) : "",
             score: question.score,
           };
         } else {
           studentAnsMap[k] = {
-            answer: JSON.parse(question.answer),
+            answer: question.answer ? JSON.parse(question.answer) : "",
             score: question.score,
           };
         }