Grey out menu if no items are present

CURA-11266

resources/qml/Menus/MaterialMenu.qml

@@ -76,6 +76,7 @@ Cura.Menu
     {
         id: genericMenu
         title: catalog.i18nc("@label:category menu label", "Generic")
+        enabled: genericMaterialsModel.items.length > 0
 
         Instantiator
         {