Disappointing. A vacuum is hardly empty. It’s a seething landscape of quantum fluctuations.
What if half the glass was truly empty? It would mean that the vacuum in that region has decayed to a ground state with zero energy or anything else. How much lower this ground state is than the vacuum we live in is unknown, but it’s undoubtedly huge. Quite possibly on the order of 10100 joules per cubic meter.
Regardless, this bubble of truly empty space would expand at the speed of light as nearby vacuum decays to the new energy state (and basically everything is a vacuum as far as this process is concerned). The sphere of expanding true vacuum would devour the Earth, the solar system, the galaxy, and eventually much of the visible universe in time. Matter as we know it could not be supported by the new vacuum.
Only those regions of the universe far enough away that they are moving apart at more than the speed of light would be spared. That’s some consolation, at least.