Use optional chaining for fetching reference

ee/app/assets/javascripts/integrations/jira/issues_show/components/sidebar/jira_issues_sidebar_root.vue

@@ -26,7 +26,7 @@ export default {
       return (this.issue?.assignees || [])[0];
     },
     reference() {
-      return this.issue.references.relative;
+      return this.issue?.references?.relative;
     },
   },
 };