Set mediaType for empty.ImageIndex in RawManifest (#1562)

Set previously unset mediaType for empty.ImageIndex so that the
mediaType will be present in the raw manifest. empty.ImageIndex.MediaType() was unimpacted because it already returned OCIImageIndex directly.

pkg/v1/empty/index.go

@@ -59,5 +59,6 @@ func (i emptyIndex) ImageIndex(v1.Hash) (v1.ImageIndex, error) {
 func base() *v1.IndexManifest {
 	return &v1.IndexManifest{
 		SchemaVersion: 2,
+		MediaType:     types.OCIImageIndex,
 	}
 }

pkg/v1/mutate/mutate_test.go

@@ -289,7 +289,7 @@ func TestAnnotations(t *testing.T) {
 	}, {
 		desc: "index",
 		in:   empty.Index,
-		want: `{"schemaVersion":2,"manifests":null,"annotations":{"foo":"bar"}}`,
+		want: `{"schemaVersion":2,"mediaType":"application/vnd.oci.image.index.v1+json","manifests":null,"annotations":{"foo":"bar"}}`,
 	}, {
 		desc: "arbitrary",
 		in:   arbitrary{},