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

Document that `Range.ToV2` method return `nil`
when called on `nil`. Document that `NewRangeFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/object/range.go

@@ -8,6 +8,8 @@ import (
 type Range object.Range
 
 // NewRangeFromV2 wraps v2 Range message to Range.
+//
+// Nil object.Range converts to nil.
 func NewRangeFromV2(rV2 *object.Range) *Range {
 	return (*Range)(rV2)
 }
@@ -18,6 +20,8 @@ func NewRange() *Range {
 }
 
 // ToV2 converts Range to v2 Range message.
+//
+// Nil Range converts to nil.
 func (r *Range) ToV2() *object.Range {
 	return (*object.Range)(r)
 }

pkg/object/range_test.go

@@ -1,6 +1,7 @@
 package object
 
 import (
+	"github.com/nspcc-dev/neofs-api-go/v2/object"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -23,3 +24,19 @@ func TestRange_SetLength(t *testing.T) {
 
 	require.Equal(t, ln, r.GetLength())
 }
+
+func TestNewRangeFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *object.Range
+
+		require.Nil(t, NewRangeFromV2(x))
+	})
+}
+
+func TestRange_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Range
+
+		require.Nil(t, x.ToV2())
+	})
+}