Error when toggling empty mobile menu

[tomusborne]

There's an error when we toggle an empty mobile menu right now: https://www.screencast.com/t/QQpCpEfLX

To reproduce, set a menu to the Primary theme location but leave it empty, then click the toggle button.

There are also errors when we resize back up to desktop while it's toggled: https://www.screencast.com/t/TyEqgZtFURc8

The obvious solution is to not show the toggle when there are no menu items, but this would break the Off-Canvas Panel option in GP Premium when people have only the toggle in the menu to open the panel. We can see this issue in the wild with our Landing site: https://gpsites.co/landing