Picking a saved login from Chrome's password autofill dispatches a plain
Event named "keydown", with no key on it. The shortcut listener passed it to
comboOf, which read the key's length and threw -- an uncaught TypeError in
the console on every sign-in. Harmless, since nothing was bound to it, but
it was noise that looks like a real fault.
comboOf now returns null for an event with no key, as it already does for a
bare modifier, so the listener stops there.
