Fix "this.groups is undefined" error when loading coursebook

aleksis/apps/alsijil/frontend/components/Coursebook.vue

@@ -76,6 +76,9 @@ export default {
     return {
       gqlQuery: documentationsForCoursebook,
       lastQuery: null,
+      // Placeholder values while query isn't completed yet
+      groups: [],
+      courses: [],
     };
   },
   apollo: {