Empty Night is a Dresden patented swear phrase, as in "Empty Night, Harry, they could have killed us!"
I should look it up and give you an actual quote, but I'm too lazy.
I just thought with all the "Nights" and "Knights" floating around and getting confused (Summer Knight/night, White Night/knight) a pseudo-curse with the word "night" in it fit right in.
Also, isn't Empty Night one of the titles Butcher hasn't written yet??? I could be totally wrong here, but without looking it up I thought you (Alan) had told us that was one of the proposed titles of the final trilogy -- or perhaps that was a joke? lol