As a very general rule, when describing a thing, it is a good idea to describe what is there, rather than what is not there. In the absence of any context whatsoever, therefore, I would choose to call the glass half filled (or half full, as tends to be more common).
However, we almost never write or speak without any context whatsoever. It may be more appropriate to emphasize the partial emptiness of the glass--for example, if there are multiple full glasses and we wish to emphasize the one that is not full. Or if the glass being filled halfway implies that it has been partially drunk--that is, that it was once full but has been half emptied. The important point to take away is that the purpose of any language is to communicate ideas, and you should choose the wording that best conveys the ideas you wish to communicate.