Main._nWorkspacesChanged was racing with Main._checkWorkspaces.
If _checkWorkspaces won the race, _workspaces was uninitialized.
Because of this, _checkWorkspaces only noticed workspaces
with windows on them, leading it to believe the last workspace wasn't
empty, and added a new, empty workspace.
https://bugzilla.gnome.org/show_bug.cgi?id=645343