add testcase in layer/empty_test.go with Metadata()

Signed-off-by: chchliang <chen.chuanliang@zte.com.cn>

layer/empty_test.go

@@ -28,6 +28,12 @@ func TestEmptyLayer(t *testing.T) {
 		t.Fatal("expected zero diffsize for empty layer")
 	}
 
+	meta, err := EmptyLayer.Metadata()
+
+	if len(meta) != 0 || err != nil {
+		t.Fatal("expected zero length metadata for empty layer")
+	}
+
 	tarStream, err := EmptyLayer.TarStream()
 	if err != nil {
 		t.Fatalf("error streaming tar for empty layer: %v", err)