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

Document that `UsedSpaceAnnouncement.ToV2` method returns
`nil` when is called on `nil`. Document that `NewAnnouncementFromV2`
method returns `nil` when is called on `nil`. Add
corresponding unit test.

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

pkg/container/announcement.go

@@ -16,6 +16,8 @@ func NewAnnouncement() *UsedSpaceAnnouncement {
 
 // NewAnnouncementFromV2 wraps protocol dependent version of
 // UsedSpaceAnnouncement message.
+//
+// Nil container.UsedSpaceAnnouncement converts to nil.
 func NewAnnouncementFromV2(v *container.UsedSpaceAnnouncement) *UsedSpaceAnnouncement {
 	return (*UsedSpaceAnnouncement)(v)
 }
@@ -53,6 +55,8 @@ func (a *UsedSpaceAnnouncement) SetUsedSpace(value uint64) {
 }
 
 // ToV2 returns protocol dependent version of UsedSpaceAnnouncement message.
+//
+// Nil UsedSpaceAnnouncement converts to nil.
 func (a *UsedSpaceAnnouncement) ToV2() *container.UsedSpaceAnnouncement {
 	return (*container.UsedSpaceAnnouncement)(a)
 }

pkg/container/announcement_test.go

@@ -8,6 +8,7 @@ import (
 	cid "github.com/nspcc-dev/neofs-api-go/pkg/container/id"
 	cidtest "github.com/nspcc-dev/neofs-api-go/pkg/container/id/test"
 	containertest "github.com/nspcc-dev/neofs-api-go/pkg/container/test"
+	containerv2 "github.com/nspcc-dev/neofs-api-go/v2/container"
 	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/stretchr/testify/require"
 )
@@ -64,3 +65,19 @@ func TestUsedSpaceEncoding(t *testing.T) {
 		require.Equal(t, a, a2)
 	})
 }
+
+func TestUsedSpaceAnnouncement_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *container.UsedSpaceAnnouncement
+
+		require.Nil(t, x.ToV2())
+	})
+}
+
+func TestNewAnnouncementFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *containerv2.UsedSpaceAnnouncement
+
+		require.Nil(t, container.NewAnnouncementFromV2(x))
+	})
+}