My beagle, Abbey, loves to bark at nothing--I'm sure it's a bunny or cat or something that she smells, but she will just stand out on the porch (only at night) and bark her head off.
A distraction is what I use--a whistle or something to get her attention, then she focuses on me. I've also seen things you can use, that send out a noise (humans can't hear it), and it distracts the dog.