(I was known as "Toad" in my past WHO/Ironworks life.)
If I am holding a cup of water and I ask you, "is the cup empty?" you will say "No, it is full of water." But if i pour out the water and I ask you again, you may say, "yes, it is empty" but, empty of what?....
My cup is empty of water, but it is not empty of air. To be empty is to be empty of something.