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.