A calorie is a unit of energy and food is fuel, so in this sense, if it doesn't have any calories it isn't a food. All drinks have some calories except plain, still water. Even black coffee has about 5 calories, though of course if you add milk and sugar you vastly increase this.

Even celery has some calories (about 6 for a typical 6-inch piece). Apparently it's not true that you burn more calories eating celery than are actually contained in the food, but you do end up with fewer calories after digesting it. The calorie loss in digesting celery is tiny, but still it is a very low calorie food that provides some nourishment and stops you eating more fattening things, so it's still worth eating.

The only thing that seems to be calorie free apart from water is dill - though as it's a herb, you wouldn't normally be eating a lot of it anyway.