Open the top up and let it evaporate. May take a few days.
I didn't know if it would be safe to add compressed air or not, but that's probably what I would do, safe or not!I would open the petcock and blow compressed air (you DO have a compressor, don't you?) in the top for a while. Then put in some soapy water, shake it around, pour it out, and do the same thing again. Blowing through enough air will evaporate it all in a pretty short time. Do all this outside, and don't smoke while you do it!