Do not crash when collector returns no data

Change-Id: Ie99d9ec4c063f88cc5dd58c844f5c2a200d35a0b
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22097
Reviewed-by: Adam Litke <alitke@redhat.com>