Added an 'Empty' choice for deck slots which aren't in use

This might fix some bugs like #404 and #408 because previously slots could internally have no deck assigned, but the UI would show that deck as being filled because there was no way to display an unused deck slot. In other words, you might think you had a deck assigned to the slot, but in fact nothing was... now at least you'll know there's no deck assigned.

(cherry picked from commit 74f4ee4)