[#302] pkg/storagegroup: Convert nil `StorageGroup` to nil message

Document that `StorageGroup.ToV2` method return `nil`
when called on `nil`. Document that `NewFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/storagegroup/storagegroup.go

@@ -11,6 +11,8 @@ import (
 type StorageGroup storagegroup.StorageGroup
 
 // NewFromV2 wraps v2 StorageGroup message to StorageGroup.
+//
+// Nil storagegroup.StorageGroup converts to nil.
 func NewFromV2(aV2 *storagegroup.StorageGroup) *StorageGroup {
 	return (*StorageGroup)(aV2)
 }
@@ -110,6 +112,8 @@ func (sg *StorageGroup) SetMembers(members []*object.ID) {
 }
 
 // ToV2 converts StorageGroup to v2 StorageGroup message.
+//
+// Nil StorageGroup converts to nil.
 func (sg *StorageGroup) ToV2() *storagegroup.StorageGroup {
 	return (*storagegroup.StorageGroup)(sg)
 }

pkg/storagegroup/storagegroup_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/nspcc-dev/neofs-api-go/pkg/storagegroup"
 	storagegrouptest "github.com/nspcc-dev/neofs-api-go/pkg/storagegroup/test"
 	refstest "github.com/nspcc-dev/neofs-api-go/pkg/test"
+	storagegroupV2 "github.com/nspcc-dev/neofs-api-go/v2/storagegroup"
 	"github.com/stretchr/testify/require"
 )
 
@@ -54,3 +55,19 @@ func TestStorageGroupEncoding(t *testing.T) {
 		require.Equal(t, sg, sg2)
 	})
 }
+
+func TestNewFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *storagegroupV2.StorageGroup
+
+		require.Nil(t, storagegroup.NewFromV2(x))
+	})
+}
+
+func TestStorageGroup_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *storagegroup.StorageGroup
+
+		require.Nil(t, x.ToV2())
+	})
+}