Revert "Empty config (#262)"

This reverts commit 069da31.

pack.go

@@ -79,15 +79,16 @@ func Pack(ctx context.Context, pusher content.Pusher, layers []ocispec.Descripto
 	if opts.ConfigDescriptor != nil {
 		configDesc = *opts.ConfigDescriptor
 	} else {
+		configBytes := []byte("{}")
 		configDesc = ocispec.Descriptor{
 			MediaType:   opts.ConfigMediaType,
-			Digest:      digest.FromBytes(nil),
-			Size:        0,
+			Digest:      digest.FromBytes(configBytes),
+			Size:        int64(len(configBytes)),
 			Annotations: opts.ConfigAnnotations,
 		}
 
 		// push config
-		if err := pusher.Push(ctx, configDesc, bytes.NewReader(nil)); err != nil && !errors.Is(err, errdef.ErrAlreadyExists) {
+		if err := pusher.Push(ctx, configDesc, bytes.NewReader(configBytes)); err != nil && !errors.Is(err, errdef.ErrAlreadyExists) {
 			return ocispec.Descriptor{}, fmt.Errorf("failed to push config: %w", err)
 		}
 	}

pack_test.go

@@ -62,15 +62,16 @@ func Test_Pack_Default(t *testing.T) {
 		t.Fatal("Oras.Pack() error =", err)
 	}
 
+	expectedConfigBytes := []byte("{}")
 	expectedManifest := ocispec.Manifest{
 		Versioned: specs.Versioned{
 			SchemaVersion: 2, // historical value. does not pertain to OCI or docker version
 		},
 		MediaType: ocispec.MediaTypeImageManifest,
 		Config: ocispec.Descriptor{
 			MediaType: MediaTypeUnknownConfig,
-			Digest:    "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
-			Size:      0,
+			Digest:    digest.FromBytes(expectedConfigBytes),
+			Size:      int64(len(expectedConfigBytes)),
 		},
 		Layers: layers,
 	}
@@ -181,15 +182,16 @@ func Test_Pack_NoLayer(t *testing.T) {
 		t.Fatal("Oras.Pack() error =", err)
 	}
 
+	expectedConfigBytes := []byte("{}")
 	expectedManifest := ocispec.Manifest{
 		Versioned: specs.Versioned{
 			SchemaVersion: 2, // historical value. does not pertain to OCI or docker version
 		},
 		MediaType: ocispec.MediaTypeImageManifest,
 		Config: ocispec.Descriptor{
 			MediaType: MediaTypeUnknownConfig,
-			Digest:    "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
-			Size:      0,
+			Digest:    digest.FromBytes(expectedConfigBytes),
+			Size:      int64(len(expectedConfigBytes)),
 		},
 		Layers: []ocispec.Descriptor{},
 	}