Menu: fix potential bug if a stand-alone non-root dialog were opened via menu_cmd...

Menu: fix potential bug if a stand-alone non-root dialog were opened via menu_cmd directmenu

Currently none of such dialogs can be opened in that way because they don't have the name field
set but if e.g. the Mutators dialog had it, then an empty menu would be displayed after closing
the Mutators dialog if opened via menu_cmd directmenu Mutators

Now the menu will be always hidden when closing a dialog that was opened via
menu_cmd directmenu <dialog_name>

This new behaviour is not applied for dialogs opened via
menu_cmd <dialog_name>