Clear old selection when wxListBox becomes empty.

Clear old selection when wxListBox becomes empty.

When UpdateOldSelections() is called from wxListBox::DoClear(), it must clear
the old selections array even for single selection list boxes, but it didn't
do this under non-MSW platforms.

Specifically check for the case of an empty listbox now and just forget the
old selections then. This fixes the problem of keeping stale old selections
and is also more efficient as we avoid the unnecessary GetSelections() call.

Really closes #14359.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71637 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775