estargz: Do not fail parsing empty layers

[ktock]

Stargz snapshotter can't perform lazy pulling for images that contains empty layers. This PR fixes this issue.

Test image

echo "FROM denoland/deno:alpine-1.43.2@sha256:f51f2853d4235c5f223015fa6f045b1978a98aa883cf15cff405bfb95656f16e" | docker buildx build --builder=container --output type=image,name=ghcr.io/ktock/deno:alpine-1.43.2-esgz-emptylayer,push=true,oci-mediatype=true,compression=estargz,force-compression=true -

This contains an empty layer "sha256:6616d0fdd6dd05b2515b5c83800740fd633a514238fe7caa3f7eeda7f89ad988".

Test command

ctr-remote i rpull ghcr.io/ktock/deno:alpine-1.43.2-esgz-emptylayer

Expected behaviour

ctr-remote i rpull succeeds with lazy pulling.

Actual behaviour (w/o the patch in this PR)

{"error":"failed to resolve layer: failed to resolve layer \"sha256:6616d0fdd6dd05b2515b5c83800740fd633a514238fe7caa3f7eeda7f89ad988\" from \"ghcr.io/ktock/deno:alpine-1.43.2-esgz-emptylayer\": failed to get root node: failed to resolve target","key":"default/3/extract-810753276-MZr_ sha256:0cdf7f72928814ea0968bde6c77c9ab437273b6b6cd8aa0e2deef5eef67fd378","level":"warning","msg":"failed to prepare remote snapshot","parent":"sha256:5d8a07f4b7477daceba36222438bf44d68ab5e44fa143767f64529020b87d181","remote-snapshot-prepared":"false","time":"2025-10-03T14:55:38.075725569Z"}

Then it fallbacks to the normal pull.