[#302] pkg/session: Convert nil `Token` to nil message

Document that `Token.ToV2` method return `nil`
when called on `nil`. Document that `NewTokenFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/session/session.go

@@ -17,6 +17,8 @@ type Token session.SessionToken
 
 // NewTokenFromV2 wraps session.SessionToken message structure
 // into Token.
+//
+// Nil session.SessionToken converts to nil.
 func NewTokenFromV2(tV2 *session.SessionToken) *Token {
 	return (*Token)(tV2)
 }
@@ -27,6 +29,8 @@ func NewToken() *Token {
 }
 
 // ToV2 converts Token to session.SessionToken message structure.
+//
+// Nil Token converts to nil.
 func (t *Token) ToV2() *session.SessionToken {
 	return (*session.SessionToken)(t)
 }

pkg/session/session_test.go

@@ -6,6 +6,7 @@ import (
 	ownertest "github.com/nspcc-dev/neofs-api-go/pkg/owner/test"
 	"github.com/nspcc-dev/neofs-api-go/pkg/session"
 	sessiontest "github.com/nspcc-dev/neofs-api-go/pkg/session/test"
+	sessionv2 "github.com/nspcc-dev/neofs-api-go/v2/session"
 	"github.com/stretchr/testify/require"
 )
 
@@ -162,3 +163,19 @@ func TestToken_Iat(t *testing.T) {
 
 	require.EqualValues(t, iat, tok.Iat())
 }
+
+func TestNewTokenFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *sessionv2.SessionToken
+
+		require.Nil(t, session.NewTokenFromV2(x))
+	})
+}
+
+func TestToken_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *session.Token
+
+		require.Nil(t, x.ToV2())
+	})
+}