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

Document that `Address.ToV2` method return `nil`
when called on `nil`. Document that `NewAddressFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/object/address.go

@@ -19,6 +19,8 @@ const (
 )
 
 // NewAddressFromV2 converts v2 Address message to Address.
+//
+// Nil refs.Address converts to nil.
 func NewAddressFromV2(aV2 *refs.Address) *Address {
 	return (*Address)(aV2)
 }
@@ -31,6 +33,8 @@ func NewAddress() *Address {
 }
 
 // ToV2 converts Address to v2 Address message.
+//
+// Nil Address converts to nil.
 func (a *Address) ToV2() *refs.Address {
 	return (*refs.Address)(a)
 }

pkg/object/address_test.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	cidtest "github.com/nspcc-dev/neofs-api-go/pkg/container/id/test"
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/stretchr/testify/require"
 )
 
@@ -84,3 +85,19 @@ func TestAddressEncoding(t *testing.T) {
 		require.Equal(t, a, a2)
 	})
 }
+
+func TestNewAddressFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *refs.Address
+
+		require.Nil(t, NewAddressFromV2(x))
+	})
+}
+
+func TestAddress_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Address
+
+		require.Nil(t, x.ToV2())
+	})
+}