engine: external scheduler dont pass empty guid

When using external balance an empty string may return resulting
in a 0000-00... guid, causing unnecessary logging of missing guid

Change-Id: I4887100e99810d3d09af90a19f116f34ede3f99e
Bug-Url: https://bugzilla.redhat.com/1017718
Signed-off-by: Noam Slomianko <nslomian@redhat.com>