Better not get technical, otherwise someone might point out all the empty space between elemental particles.

One could go even further and say that since everything is an illusion, there is no glass in the first place.

We could also view things like Discord: The chocolate milk is always full.

Let's stick with Pinkie Pie.

"The glass is half full" is also a near homonym of "The glasses are full". So now there's more than one glass, and they're both full. Win win!