lookingGlass: Handle undefined in objectToString()

Fallout from commit 9d941f8202: Template strings handle undefined
values just fine, the replacement does not. Fix that.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1020

js/ui/lookingGlass.js

@@ -244,7 +244,7 @@ function objectToString(o) {
         // special case this since the default is way, way too verbose
         return '<js function>';
     } else {
-        return o.toString();
+        return o !== undefined ? o.toString() : 'undefined';
     }
 }