oras discover should show a placeholder of artifactType if it's not presented in an index referrer

[qweeah]

What is the version of your ORAS CLI

1.1.0

What would you like to be added?

Currently oras discover shows an empty node when artifactType if it's not presented in an index referrer, like

✗ oras discover -o tree localhost:5000/test:multi
localhost:7000/command/artifacts@sha256:e2bfc9cc6a84ec2d7365b5a28c6bc5806b7fa581c9ad7883be955a64e3cc034f
├── referrer/index
│   └── sha256:d37baf66300b9006b0f4c7102075d56b970fbf910be5c6bca07fdbb000dfa383
└── 
    └── sha256:7679bc22c33b87aa345c6950a993db98a6df7a6cc77a35c388908a3

Should use a placeholder for the empty artifactType, like <nil> since fmt.Println(nil) prints <nil>

✗ oras discover -o tree localhost:5000/test:multi
localhost:7000/command/artifacts@sha256:e2bfc9cc6a84ec2d7365b5a28c6bc5806b7fa581c9ad7883be955a64e3cc034f
├── referrer/index
│   └── sha256:d37baf66300b9006b0f4c7102075d56b970fbf910be5c6bca07fdbb000dfa383
└── <nil>
    └── sha256:7679bc22c33b87aa345c6950a993db98a6df7a6cc77a35c388908a3

Why is this needed for ORAS?

Showing an empty node makes no sense and will confuse our user.

Are you willing to submit PRs to contribute to this feature?

• Yes, I am willing to implement it.

[Wwwsylvia]

<nil> makes me feel like the node is nil, but not artifactType is nil.
How about something like <unknown> or <unknown artifact>?

$ oras discover -o tree localhost:5000/test:multi
localhost:7000/command/artifacts@sha256:e2bfc9cc6a84ec2d7365b5a28c6bc5806b7fa581c9ad7883be955a64e3cc034f
├── referrer/index
│   └── sha256:d37baf66300b9006b0f4c7102075d56b970fbf910be5c6bca07fdbb000dfa383
└── <unknown>
    └── sha256:7679bc22c33b87aa345c6950a993db98a6df7a6cc77a35c388908a3

[wangxiaoxuan273]

I'd like to work on this item