[#302] pkg/owner: Convert nil `ID` to nil message

Document that `ID.ToV2` method return `nil`
when called on `nil`. Document that `NewIDFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/owner/id.go

@@ -16,6 +16,8 @@ type ID refs.OwnerID
 var errInvalidIDString = errors.New("incorrect format of the string owner ID")
 
 // NewIDFromV2 wraps v2 OwnerID message to ID.
+//
+// Nil refs.OwnerID converts to nil.
 func NewIDFromV2(idV2 *refs.OwnerID) *ID {
 	return (*ID)(idV2)
 }
@@ -33,6 +35,8 @@ func (id *ID) SetNeo3Wallet(v *NEO3Wallet) {
 }
 
 // ToV2 returns the v2 owner ID message.
+//
+// Nil ID converts to nil.
 func (id *ID) ToV2() *refs.OwnerID {
 	return (*refs.OwnerID)(id)
 }

pkg/owner/id_test.go

@@ -7,6 +7,7 @@ import (
 	"github.com/mr-tron/base58"
 	. "github.com/nspcc-dev/neofs-api-go/pkg/owner"
 	ownertest "github.com/nspcc-dev/neofs-api-go/pkg/owner/test"
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/nspcc-dev/neofs-crypto/test"
 	"github.com/stretchr/testify/require"
 )
@@ -99,3 +100,19 @@ func TestID_Equal(t *testing.T) {
 		ownertest.GenerateFromBytes(data3),
 	))
 }
+
+func TestNewIDFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *refs.OwnerID
+
+		require.Nil(t, NewIDFromV2(x))
+	})
+}
+
+func TestID_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *ID
+
+		require.Nil(t, x.ToV2())
+	})
+}