[#209] Support nil value in SplitID

SplitID is not set on non-split and virtual
objects, so we should support this state in
library.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/object/splitid.go

@@ -42,17 +42,27 @@ func (id *SplitID) Parse(s string) (err error) {
 }
 
 // String returns UUIDv4 string representation of SplitID.
-func (id SplitID) String() string {
+func (id *SplitID) String() string {
+	if id == nil {
+		return ""
+	}
+
 	return id.uuid.String()
 }
 
 // SetUUID sets pre created UUID structure as SplitID.
 func (id *SplitID) SetUUID(v uuid.UUID) {
+	if id != nil {
 		id.uuid = v
+	}
 }
 
 // ToV2 converts SplitID to a representation of SplitID in neofs-api v2.
-func (id SplitID) ToV2() []byte {
+func (id *SplitID) ToV2() []byte {
+	if id == nil {
+		return nil
+	}
+
 	data, _ := id.uuid.MarshalBinary() // err is always nil
 
 	return data

pkg/object/splitid_test.go

@@ -35,4 +35,13 @@ func TestSplitID(t *testing.T) {
 
 		require.Equal(t, newUUID.String(), id.String())
 	})
+
+	t.Run("nil value", func(t *testing.T) {
+		var newID *object.SplitID
+
+		require.NotPanics(t, func() {
+			require.Nil(t, newID.ToV2())
+			require.Equal(t, "", newID.String())
+		})
+	})
 }