bugfix: cache empty layer for docker schema1 image
bugfix: cache empty layer for docker schema1 image

containerd should cache empty label for docker schema1 image.

if not, the original empty layer will be non-empty layer and the image
config will be changed too. in this case, the image ID will be changed.

check the blob empty label to avoid changing image ID when repull docker
schema1 image.

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

thaJeztah/containerd,estesp/containerd,containerd/containerd,containerd/containerd,dmcgowan/containerd,vdemeester/containerd,dmcgowan/containerd,miaoyq/containerd,vdemeester/containerd,thaJeztah/containerd,miaoyq/containerd,mikebrow/containerd,crosbymichael/containerd,mikebrow/containerd,estesp/containerd,crosbymichael/containerd