Empty Simbad queries return "None"

With this update, Simbad queries which have no result return "None", instead of an empty `astropy.table.Table`.  This fixes https://github.com/astropy/astroquery/issues/368 .  This reveals an existing bug: https://github.com/astropy/astroquery/issues/371
Yeah, I just meant a glob to represent the different result classes.