The image inspect response has various fields that are either optional,
or only used if the image was built using the deprecated legacy builder.
This patch marks them as "omitempty" to omit them from the response if
not set:

- The `Parent` field is only used for the legacy builder, and only set for
  images that are built locally (i.e., not persisted when pulling an image).
- The `Comment` field is optional, and may not be set, depending on how the
  image is produced.
- The `DockerVersion` field is only set when building images with the legacy
  builder, and empty in most cases.
- The `Author` field can be set through the `MAINTAINER` instruction in
  Dockerfiles, and through the `--author` option on `docker commit`, but
  is optional, and won't be set in most situations.

With this patch:

On API v1.52

    DOCKER_API_VERSION=v1.52 docker inspect busybox
    [
        {
            "Id": "sha256:d82f458899c9696cb26a7c02d5568f81c8c8223f8661bb2a7988b269c8b9051e",
            "RepoTags": ["busybox:latest"],
            "RepoDigests": ["busybox@sha256:d82f458899c9696cb26a7c02d5568f81c8c8223f8661bb2a7988b269c8b9051e"],
            "Created": "2024-09-26T21:31:42Z",
            "Config": {
                "Env": ["PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"],
                "Cmd": ["sh"]
            },
            "Architecture": "arm64",
            "Variant": "v8",
            "Os": "linux",
            "Size": 1913388,
            "RootFS": {
                "Type": "layers",
                "Layers": ["sha256:6aba5e0d32d91e3e923854dcb30588dc4112bfa1dae82b89535ad31d322a7b19"]
            },
            "Metadata": {
                "LastTagTime": "2025-10-03T22:24:18.440035424Z"
            },
            "Descriptor": {
                "mediaType": "application/vnd.oci.image.index.v1+json",
                "digest": "sha256:d82f458899c9696cb26a7c02d5568f81c8c8223f8661bb2a7988b269c8b9051e",
                "size": 9535
            }
        }
    ]

On API v1.51 or lower:

    DOCKER_API_VERSION=v1.51 docker inspect busybox
    [
        {
            "Architecture": "arm64",
            "Author": "",
            "Cmd": null,
            "Comment": "",
            "Config": {
                "Cmd": ["sh"],
                "Env": ["PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"]
            },
            "Created": "2024-09-26T21:31:42Z",
            "Descriptor": {
                "digest": "sha256:d82f458899c9696cb26a7c02d5568f81c8c8223f8661bb2a7988b269c8b9051e",
                "mediaType": "application/vnd.oci.image.index.v1+json",
                "size": 9535
            },
            "DockerVersion": "",
            "Entrypoint": null,
            "Env": null,
            "Id": "sha256:d82f458899c9696cb26a7c02d5568f81c8c8223f8661bb2a7988b269c8b9051e",
            "Labels": null,
            "Metadata": {
                "LastTagTime": "2025-10-03T22:24:18.440035424Z"
            },
            "OnBuild": null,
            "Os": "linux",
            "Parent": "",
            "RepoDigests": ["busybox@sha256:d82f458899c9696cb26a7c02d5568f81c8c8223f8661bb2a7988b269c8b9051e"],
            "RepoTags": ["busybox:latest"],
            "RootFS": {
                "Layers": ["sha256:6aba5e0d32d91e3e923854dcb30588dc4112bfa1dae82b89535ad31d322a7b19"],
                "Type": "layers"
            },
            "Size": 1913388,
            "User": "",
            "Variant": "v8",
            "Volumes": null,
            "WorkingDir": ""
        }
    ]

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>