feat: oras discover should show a placeholder of artifactType if it's…

… not presented

Signed-off-by: Xiaoxuan Wang <[email protected]>

cmd/oras/root/discover.go

@@ -150,6 +150,9 @@ func fetchAllReferrers(ctx context.Context, repo oras.ReadOnlyGraphTarget, desc
 
 	for _, r := range results {
 		// Find all indirect referrers
+		if r.ArtifactType == "" {
+			r.ArtifactType = "<unknown>"
+		}
 		referrerNode := node.AddPath(r.ArtifactType, r.Digest)
 		if opts.Verbose {
 			for k, v := range r.Annotations {