[#302] pkg/audit: Convert nil Result to nil message

Document that `Result.ToV2` method returns
`nil` when is called on `nil`. Document that
`NewResultFromV2` method returns `nil` when
is called on `nil`. Add
corresponding unit test.

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

pkg/audit/result.go

@@ -12,6 +12,8 @@ import (
 type Result audit.DataAuditResult
 
 // NewFromV2 wraps v2 DataAuditResult message to Result.
+//
+// Nil audit.DataAuditResult converts to nil.
 func NewResultFromV2(aV2 *audit.DataAuditResult) *Result {
 	return (*Result)(aV2)
 }
@@ -25,6 +27,8 @@ func NewResult() *Result {
 }
 
 // ToV2 converts Result to v2 DataAuditResult message.
+//
+// Nil Result converts to nil.
 func (r *Result) ToV2() *audit.DataAuditResult {
 	return (*audit.DataAuditResult)(r)
 }

pkg/audit/result_test.go

@@ -9,6 +9,7 @@ import (
 	cidtest "github.com/nspcc-dev/neofs-api-go/pkg/container/id/test"
 	"github.com/nspcc-dev/neofs-api-go/pkg/object"
 	objecttest "github.com/nspcc-dev/neofs-api-go/pkg/object/test"
+	auditv2 "github.com/nspcc-dev/neofs-api-go/v2/audit"
 	"github.com/stretchr/testify/require"
 )
 
@@ -91,3 +92,19 @@ func TestStorageGroupEncoding(t *testing.T) {
 		require.Equal(t, r, r2)
 	})
 }
+
+func TestResult_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *audit.Result
+
+		require.Nil(t, x.ToV2())
+	})
+}
+
+func TestNewResultFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *auditv2.DataAuditResult
+
+		require.Nil(t, audit.NewResultFromV2(x))
+	})
+}