If "full" is 100% of something and "empty" is 0% of something, then half of 0% is still 0%. "Half-empty" is more easily stated as simply "empty". It can't be any sort of empty if there is some quantity of the material in question inside the glass. So I say half-full, but never half-empty unless it is also completely empty. I completely dorked-out that answer, eh?
WTF cares? Drink it and pour another *full* one. Alternatively, neither. It is a 2D digital image therefore it has no volume, per se.