Handle null message (#455)

If the message was not found on a notification, don't try to display
anything.

- Refs readthedocs/readthedocs.org#11542

Author: agjohnson

readthedocsext/theme/static/readthedocsext/theme/js/site.js

@@ -27,7 +27,7 @@ export class NotificationElement extends LightDOMElement {
   };
 
   render() {
-    if (this.notification === undefined) {
+    if (this.notification === undefined || this.notification.message === null) {
       return nothing;
     }