[#302] pkg/object: Convert nil ID to nil message

Document that `ID.ToV2` method return `nil`
when called on `nil`. Document that `NewIDFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/object/id.go

@@ -16,6 +16,8 @@ type ID refs.ObjectID
 var errInvalidIDString = errors.New("incorrect format of the string object ID")
 
 // NewIDFromV2 wraps v2 ObjectID message to ID.
+//
+// Nil refs.ObjectID converts to nil.
 func NewIDFromV2(idV2 *refs.ObjectID) *ID {
 	return (*ID)(idV2)
 }
@@ -41,6 +43,8 @@ func (id *ID) Equal(id2 *ID) bool {
 }
 
 // ToV2 converts ID to v2 ObjectID message.
+//
+// Nil ID converts to nil.
 func (id *ID) ToV2() *refs.ObjectID {
 	return (*refs.ObjectID)(id)
 }

pkg/object/id_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/mr-tron/base58"
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/stretchr/testify/require"
 )
 
@@ -107,3 +108,19 @@ func TestObjectIDEncoding(t *testing.T) {
 		require.Equal(t, id, a2)
 	})
 }
+
+func TestNewIDFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *refs.ObjectID
+
+		require.Nil(t, NewIDFromV2(x))
+	})
+}
+
+func TestID_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *ID
+
+		require.Nil(t, x.ToV2())
+	})
+}