优化type check

src/features/student/inspect/MarkBody.vue

@@ -46,9 +46,9 @@ interface SliceImage {
   originalImageHeight: number;
 }
 
-const { usingImage } = withDefaults(
+const { usingImage = "sliceUrls" } = withDefaults(
   defineProps<{
-    usingImage: "sheetUrls" | "sliceUrls";
+    usingImage?: "sheetUrls" | "sliceUrls";
   }>(),
   {
     usingImage: "sliceUrls",