If I can't see the glass there's no way for me to verify anything, therefore I do not possess the neccesary information to answer this question. If there is indeed no glass, however, I couldn't verify anything anyway, because you cannot prove a nonexistance. Because of this, until other information is provided, I shall think the most likely to be correct answer is: there is no glass.
However, I do see a mug right in front of me which is a little less than half full.