Have a default SalMenu::GetSystemMenuData implementation
that does nothing instead of being purely virtual
and all subclasses except WinSalMenu::GetSystemMenuData
having to override it to do nothing.
Change-Id: Ia47af286f0fd3c1e3c6a00fff4512c9334fd6e9b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/177660
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins
