Cage's point is well-documented: essentially it's a musical interpretation of the old saying Nature abhors a vacuum. I don't know whether anyone has ever presented a completely blank canvas, but it would make the same statement - there is still something there.
It's interesting from the point of view of quantum mechanics too, which postulates that particle-antiparticle pairs can spontaneously appear out of nowhere for infinitesimal periods of time. (In essence, conservation of energy only holds at a statistical level, and can be broken within the confines of the Uncertainty Principle). If you took away all the matter in the Universe, it still wouldn't really be empty space.