Put manifest without subject fails

[sajayantony]

Once the mediaType was removed the distribution fork could not handle manifests without subjects

See manifest below -

{
    "artifactType": "test",
    "blobs": [
        {
            "mediaType": "application/vnd.oci.image.layer.v1.tar",
            "digest": "sha256:641e4de3c3a30dd1abab107fad45eef2594e200a7f2085e648035d3efe43cf44",
            "size": 4235802,
            "urls": [
                "https://github.com/sajayantony/oras/releases/download/v0.5.4-alpha/oras_0.5.4-alpha_linux_amd64.tar.gz"
            ]
        }
    ]
}
```

Also if there are url fields should we validate that the blob exists? 

[shizhMSFT]

@m5i-work Could you work on this?

[m5i-work]

@shizhMSFT Sure, let me take a look.

[shizhMSFT]

Pending on oras-project/artifacts-spec#103

[shizhMSFT]

Resolved by #19