When opening a large number of windows, the computed space and scale for
a layout can become negative due to the per-row/per-column spacing. This
is smaller than the initial values of lastSpace and lastSpace, leading
to a null return which then causes all sorts of other issues resulting
in the workspace becoming invisible.
This change ensures that the function always returns a layout, even if
it may look a bit broken and does not conform to the scale/space
requirements which are impossible to fulfill for the given number of
windows. It's better than displaying nothing, since it allows users to
move/close windows and restore this to a more usable state.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3730
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1685>