make sure we have a default handling the quit command, see #12402

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74061 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
using an empty default menu bar when no menubar is available, see #12402

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74062 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775