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

Document that `SplitInfo.ToV2` method return `nil`
when called on `nil`. Document that `NewSplitInfoFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/object/splitinfo.go

@@ -6,6 +6,9 @@ import (
 
 type SplitInfo object.SplitInfo
 
+// NewSplitInfoFromV2 wraps v2 SplitInfo message to SplitInfo.
+//
+// Nil object.SplitInfo converts to nil.
 func NewSplitInfoFromV2(v2 *object.SplitInfo) *SplitInfo {
 	return (*SplitInfo)(v2)
 }
@@ -14,6 +17,9 @@ func NewSplitInfo() *SplitInfo {
 	return NewSplitInfoFromV2(new(object.SplitInfo))
 }
 
+// ToV2 converts SplitInfo to v2 SplitInfo message.
+//
+// Nil SplitInfo converts to nil.
 func (s *SplitInfo) ToV2() *object.SplitInfo {
 	return (*object.SplitInfo)(s)
 }

pkg/object/splitinfo_test.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/nspcc-dev/neofs-api-go/pkg/object"
+	objv2 "github.com/nspcc-dev/neofs-api-go/v2/object"
 	"github.com/stretchr/testify/require"
 )
 
@@ -51,3 +52,19 @@ func generateID() *object.ID {
 
 	return id
 }
+
+func TestNewSplitInfoFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *objv2.SplitInfo
+
+		require.Nil(t, object.NewSplitInfoFromV2(x))
+	})
+}
+
+func TestSplitInfo_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *object.SplitInfo
+
+		require.Nil(t, x.ToV2())
+	})
+}