workspace: Force creating new layout when there's no allocation yet

workspace: Force creating new layout when there's no allocation yet

Commit ba23279f1fd3820fb81570b1b4a03048e5a7a3bc was aimed at fixing a
bug where the layout is frozen so early that we don't have an existing
one to re-use, because no allocation cycle happened yet. It tried to fix
that by forcing the creation of a layout when needsLayout === true,
this turned out to be a bit too much, as it also forced creating a new
layout when the layout was frozen after closing a window (which would
set needsLayout to true).

To fix this regression and still avoid reintroducing the bug the
previous commit fixed, tighten the check a bit and only force creating
the new layout when there's no allocation yet. This makes freezing the
layout after closing a window work again.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2236>