As I understand it, dielectric breakdown voltage is the energy necessary to disrupt an insulator into a conductor. Therefore, if you have a perfect vacuum, there is no matter to disrupt. But, a high enough voltage could still create electron-positron pairs out of the vacuum and therefore make a conductor in something empty. It's called schwinger effect. It is only a prediction of quantum electrodynamics.

So if there is nothing there, electricity could pass through when "matter" (here electron and positron) are created in it.