Allow Restricted Notebooks to export text (#6542)

* Refactor string to stream

* add to restricted notebooks and allow for blank users

* forgot to add notebook

* use better types and fix test

* move streamToString

* add export group

* catch blank pages

src/plugins/notebook/utils/notebook-entries.js

@@ -2,7 +2,7 @@ import objectLink from '../../../ui/mixins/object-link';
 import { v4 as uuid } from 'uuid';
 
 async function getUsername(openmct) {
-    let username = '';
+    let username = null;
 
     if (openmct.user.hasProvider()) {
         const user = await openmct.user.getCurrentUser();