Fix restorer

Signed-off-by: Natalie Arellano <narellano@vmware.com>

buildpack/layertypes/types.go

@@ -1,13 +1,13 @@
 package layertypes
 
 type LayerMetadataFile struct {
-	BOM    BOM         `json:"bom,omitempty" toml:"bom,omitempty"`
+	BOM    *BOM        `json:"bom,omitempty" toml:"bom,omitempty"`
 	Data   interface{} `json:"data" toml:"metadata"`
 	Build  bool        `json:"build" toml:"build"`
 	Launch bool        `json:"launch" toml:"launch"`
 	Cache  bool        `json:"cache" toml:"cache"`
 }
 
 type BOM struct {
-	SHA string `json:"sha" toml:"sha"`
+	SHA string `json:"sha,omitempty" toml:"sha,omitempty"`
 }

cache.go

@@ -3,6 +3,7 @@ package lifecycle
 import (
 	"github.com/pkg/errors"
 
+	"github.com/buildpacks/lifecycle/buildpack/layertypes"
 	"github.com/buildpacks/lifecycle/platform"
 )
 
@@ -42,7 +43,12 @@ func (e *Exporter) Cache(layersDir string, cacheStore Cache) error {
 			origLayerMetadata := origMeta.MetadataForBuildpack(bp.ID).Layers[layer.name()]
 			sbomLayer := bpDir.newSBOMLayerFor(layer.Identifier())
 			if sbomLayer != nil {
-				if lmd.BOM.SHA, err = e.addOrReuseSBOMLayer(cacheStore, *sbomLayer, origLayerMetadata.BOM.SHA); err != nil {
+				var origSHA string
+				if origLayerMetadata.BOM != nil {
+					origSHA = origLayerMetadata.BOM.SHA
+				}
+				lmd.BOM = &layertypes.BOM{}
+				if lmd.BOM.SHA, err = e.addOrReuseSBOMLayer(cacheStore, *sbomLayer, origSHA); err != nil {
 					e.Logger.Warnf("Failed to cache layer '%s': %s", sbomLayer.identifier, err) // TODO: check
 					continue
 				}