Resolve "Opening participations dialog in coursebook still does additionial GraphQL queries"

Closes #334 (closed)

aleksis/apps/alsijil/frontend/components/extra_marks/ExtraMarkButtons.vue

@@ -13,7 +13,7 @@ export default {
       query: extraMarks,
       update: (data) => data.items,
       skip() {
-        return this.customExtraMarks > 0;
+        return this.customExtraMarks.length > 0;
       },
     },
   },