systemActions: Fix initial visibility of logout action

systemActions: Fix initial visibility of logout action

When `_updateLogout()` is called on login, the AccountsService user
might not have been loaded yet. This would have resulted in all values
being null/false, including `local_account`. Since `!localAccount` is
one of the cases where the action is available, this would have wrongly
resulted in the item being shown when not intended to be shown. But only
until something triggers another call of `_updateLogout()`, such as
locking/unlocking the session.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/9100
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4121>
(cherry picked from commit 6f5f3c1f18a3597df04f831d4a8e1cdba2c0ecea)