Don't output the extra spacing around the images when none of the
top-level image entries has any children.
This makes the list look better when ran against the graphdrivers image
store.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 7b91647943)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>