[#302] pkg/version: Convert nil `Version` to nil message

Document that `Version.ToV2` method return `nil`
when called on `nil`. Document that `NewVersionFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/version.go

@@ -12,6 +12,8 @@ type Version refs.Version
 const sdkMjr, sdkMnr = 2, 6
 
 // NewVersionFromV2 wraps v2 Version message to Version.
+//
+// Nil refs.Version converts to nil.
 func NewVersionFromV2(v *refs.Version) *Version {
 	return (*Version)(v)
 }
@@ -58,6 +60,8 @@ func (v *Version) SetMinor(val uint32) {
 }
 
 // ToV2 converts Version to v2 Version message.
+//
+// Nil Version converts to nil.
 func (v *Version) ToV2() *refs.Version {
 	return (*refs.Version)(v)
 }

pkg/version_test.go

@@ -3,10 +3,11 @@ package pkg
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/stretchr/testify/require"
 )
 
-func TestNewVersionFromV2(t *testing.T) {
+func TestNewVersion(t *testing.T) {
 	v := NewVersion()
 
 	var mjr, mnr uint32 = 1, 2
@@ -88,3 +89,19 @@ func TestVersionEncoding(t *testing.T) {
 		require.Equal(t, v, v2)
 	})
 }
+
+func TestNewVersionFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *refs.Version
+
+		require.Nil(t, NewVersionFromV2(x))
+	})
+}
+
+func TestVersion_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Version
+
+		require.Nil(t, x.ToV2())
+	})
+}