make empty root a variable

gateway/blocks_backend.go

@@ -283,6 +283,11 @@ func (bb *BlocksBackend) Head(ctx context.Context, path ImmutablePath) (ContentP
 	return ContentPathMetadata{}, nil, fmt.Errorf("unsupported UnixFS file type")
 }
 
+// emptyRoot is a CAR root with the empty identity CID. CAR files are recommended
+// to always include a CID in their root, even if it's just the empty CID.
+// https://ipld.io/specs/transport/car/carv1/#number-of-roots
+var emptyRoot = []cid.Cid{cid.MustParse("bafkqaaa")}
+
 func (bb *BlocksBackend) GetCAR(ctx context.Context, p ImmutablePath, params CarParams) (ContentPathMetadata, io.ReadCloser, error) {
 	pathMetadata, err := bb.ResolvePath(ctx, p)
 	if err != nil {
@@ -292,7 +297,7 @@ func (bb *BlocksBackend) GetCAR(ctx context.Context, p ImmutablePath, params Car
 		}
 
 		var buf bytes.Buffer
-		cw, err := storage.NewWritable(&buf, []cid.Cid{cid.MustParse("bafkqaaa")}, car.WriteAsCarV1(true))
+		cw, err := storage.NewWritable(&buf, emptyRoot, car.WriteAsCarV1(true))
 		if err != nil {
 			return ContentPathMetadata{}, nil, err
 		}