Improve empty() file checker for pycbc io
This PR patches the table.io.pycbc.empty_hdf5_file
to return True
(i.e. 'empty') if there isn't a group for the given ifo
. Previously if there were groups for other IFOs, the function would then try to list()
the given one, which meant if it didn't exist, you would end up with a KeyError
.