[#302] pkg/signature: Convert nil `Signature` to nil message

Document that `Signature.ToV2` method return `nil`
when called on `nil`. Document that `NewSignatureFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/signature.go

@@ -8,6 +8,8 @@ import (
 type Signature refs.Signature
 
 // NewSignatureFromV2 wraps v2 Signature message to Signature.
+//
+// Nil refs.Signature converts to nil.
 func NewSignatureFromV2(sV2 *refs.Signature) *Signature {
 	return (*Signature)(sV2)
 }
@@ -39,6 +41,9 @@ func (s *Signature) SetSign(v []byte) {
 	(*refs.Signature)(s).SetSign(v)
 }
 
+// ToV2 converts Signature to v2 Signature message.
+//
+// Nil Signature converts to nil.
 func (s *Signature) ToV2() *refs.Signature {
 	return (*refs.Signature)(s)
 }

pkg/signature_test.go

@@ -1,6 +1,7 @@
 package pkg
 
 import (
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -31,3 +32,19 @@ func TestSignatureEncoding(t *testing.T) {
 		require.Equal(t, s, s2)
 	})
 }
+
+func TestNewSignatureFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *refs.Signature
+
+		require.Nil(t, NewSignatureFromV2(x))
+	})
+}
+
+func TestSignature_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Signature
+
+		require.Nil(t, x.ToV2())
+	})
+}