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

Document that `SplitID.ToV2` method return `nil`
when called on `nil`. Document that `NewSplitIDFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/object/splitid.go

@@ -18,7 +18,13 @@ func NewSplitID() *SplitID {
 
 // NewSplitIDFromV2 returns parsed UUID from bytes.
 // If v is invalid UUIDv4 byte sequence, then function returns nil.
+//
+// Nil converts to nil.
 func NewSplitIDFromV2(v []byte) *SplitID {
+	if v == nil {
+		return nil
+	}
+
 	id := uuid.New()
 
 	err := id.UnmarshalBinary(v)
@@ -58,6 +64,8 @@ func (id *SplitID) SetUUID(v uuid.UUID) {
 }
 
 // ToV2 converts SplitID to a representation of SplitID in neofs-api v2.
+//
+// Nil SplitID converts to nil.
 func (id *SplitID) ToV2() []byte {
 	if id == nil {
 		return nil

pkg/object/splitid_test.go

@@ -46,3 +46,18 @@ func TestSplitID(t *testing.T) {
 	})
 }
 
+func TestSplitID_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *object.SplitID
+
+		require.Nil(t, x.ToV2())
+	})
+}
+
+func TestNewIDFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x []byte
+
+		require.Nil(t, object.NewSplitIDFromV2(x))
+	})
+}