Restore compatibility with PrettyTable < 0.7.2
Restore compatibility with PrettyTable < 0.7.2

PrettyTable 0.7.2 defaults to print a table even if
the resultset is empty. Restore previous behavior
to be in line with older prettytable releases.

Change-Id: Idd1d0c2c2f41966076517cbc8819ba9a5e097306

jamielennox/keystoneauth,sileht/keystoneauth,citrix-openstack-build/keystoneauth