When you refer to something that is not there, you can refer to its vacuity. A stern teacher who is criticizing an empty-headed English essay might talk about its vacuity, or the absence of any real substance.
This abstract noun is derived from the adjective vacuous, which means "like or of a vacuum." Vacuity is rarely used to refer to actual vacuums; it is instead used to disparage arguments, morals, and undertakings that lack proper substance. These transferred meanings are more common for the adjective as well. Use them both sparingly: they are nearly always a marker that you have your judgment hat on!