listviews: Reset scrollbar adjustment when list is empty

listviews: Reset scrollbar adjustment when list is empty

In a list with a visible scrollbar, the scrollbar usually becomes
invisible when the numbers of items is less than the required amount
to scroll.  If, however, the list is emptied all at once,
the scrollbar remains.  This happens because when there's an empty
list gtk_list_view_size_allocate() returns early before the scrollbar
adjustment is updated.

Given that the list is empty, simply reset the adjustment values
to zero.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/4370