If I use Zola's guest addressing feature, but don't have addresses for all of my envelopes, will the rest of my envelopes be blank?

Great question!  Yes, any "extra" cards in your order without guest addresses on them will print as a blank envelope.  For example, if you order 100 cards, but only have 85 guests, you will receive 85 envelopes/postcards with your guests' addresses on them and 15 cards with just your return address printed.