Avoid situations with no menu in headerbar

Some shells might want to show menu button outside of the app. We do not support that.

src/headerbarBase.js

@@ -151,7 +151,7 @@ class ClapperHeaderBarBase extends Gtk.Box
 
         for(let name of layoutArr) {
             /* Menu might be named "appmenu" */
-            if(!menuAdded && name === 'appmenu')
+            if(!menuAdded && (!name || name === 'appmenu'))
                 name = 'menu';
 
             const widget = this[`${name}Widget`];