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

Document that `Attribute.ToV2` method return `nil`
when called on `nil`. Document that `NewAttributeFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/object/attribute.go

@@ -8,6 +8,8 @@ import (
 type Attribute object.Attribute
 
 // NewAttributeFromV2 wraps v2 Attribute message to Attribute.
+//
+// Nil object.Attribute converts to nil.
 func NewAttributeFromV2(aV2 *object.Attribute) *Attribute {
 	return (*Attribute)(aV2)
 }
@@ -40,6 +42,8 @@ func (a *Attribute) SetValue(v string) {
 }
 
 // ToV2 converts Attribute to v2 Attribute message.
+//
+// Nil Attribute converts to nil.
 func (a *Attribute) ToV2() *object.Attribute {
 	return (*object.Attribute)(a)
 }

pkg/object/attribute_test.go

@@ -3,6 +3,7 @@ package object
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/object"
 	"github.com/stretchr/testify/require"
 )
 
@@ -47,3 +48,19 @@ func TestAttributeEncoding(t *testing.T) {
 		require.Equal(t, a, a2)
 	})
 }
+
+func TestNewAttributeFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *object.Attribute
+
+		require.Nil(t, NewAttributeFromV2(x))
+	})
+}
+
+func TestAttribute_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Attribute
+
+		require.Nil(t, x.ToV2())
+	})
+}