fix type error

src/features/OnlinePractice/ExamPaperPreview.vue

@@ -165,7 +165,7 @@ function parseRightAnswer(
 }
 function parseStudentAnswer(
   questionType: string,
-  studentAnswer: string,
+  studentAnswer: string | null,
   optionPermutation: number[] | undefined
 ) {
   if (!studentAnswer) return "";
@@ -186,7 +186,7 @@ function parseStudentAnswer(
 }
 function equalAnswer(
   questionType: string,
-  studentAnswer: string,
+  studentAnswer: string | null,
   rightAnswer: string[] | undefined
 ) {
   if (!rightAnswer) return null;