Fix return value of "image set" command
Fix return value of "image set" command

"image set" command should return None. But in one path,
it returns ({}, {}). This patch fixes this.

Change-Id: I3847e661cb7e89863921a3f0a859d9b1a8077ede

redhat-openstack/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,dtroyer/python-openstackclient,redhat-openstack/python-openstackclient,openstack/python-openstackclient