imagebuildah: recognize cache images for layerless images
imagebuildah: recognize cache images for layerless images

Update cache logic to recognize images that don't include any layers.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #1580
Approved by: rhatdan

nalind/buildah-1,projectatomic/buildah,nalind/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,projectatomic/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah,nalind/buildah-1,projectatomic/buildah