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

Document that `Attribute.ToV2` method returns
`nil` when is called on `nil`. Add
corresponding unit test.

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

pkg/container/attribute.go

@@ -29,10 +29,17 @@ func (a *Attribute) Value() string {
 	return (*container.Attribute)(a).GetValue()
 }
 
+// NewAttributeFromV2 wraps protocol dependent version of
+// Attribute message.
+//
+// Nil container.Attribute converts to nil.
 func NewAttributeFromV2(v *container.Attribute) *Attribute {
 	return (*Attribute)(v)
 }
 
+// ToV2 converts Attribute to v2 Attribute message.
+//
+// Nil Attribute converts to nil.
 func (a *Attribute) ToV2() *container.Attribute {
 	return (*container.Attribute)(a)
 }

pkg/container/attribute_test.go

@@ -4,10 +4,17 @@ import (
 	"testing"
 
 	"github.com/nspcc-dev/neofs-api-go/pkg/container"
+	containerv2 "github.com/nspcc-dev/neofs-api-go/v2/container"
 	"github.com/stretchr/testify/require"
 )
 
 func TestAttribute(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *container.Attribute
+
+		require.Nil(t, x.ToV2())
+	})
+
 	const (
 		key   = "key"
 		value = "value"
@@ -82,3 +89,11 @@ func TestAttributes(t *testing.T) {
 		}
 	})
 }
+
+func TestNewAttributeFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *containerv2.Attribute
+
+		require.Nil(t, container.NewAttributeFromV2(x))
+	})
+}