[#302] pkg/xheader: Convert nil `XHeader` to nil message

Document that `XHeader.ToV2` method return `nil`
when called on `nil`. Document that `NewXHeaderFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/xheader.go

@@ -8,6 +8,8 @@ import (
 type XHeader session.XHeader
 
 // NewXHeaderFromV2 wraps v2 XHeader message to XHeader.
+//
+// Nil session.XHeader converts to nil.
 func NewXHeaderFromV2(v *session.XHeader) *XHeader {
 	return (*XHeader)(v)
 }
@@ -18,6 +20,8 @@ func NewXHeader() *XHeader {
 }
 
 // ToV2 converts XHeader to v2 XHeader message.
+//
+// Nil XHeader converts to nil.
 func (x *XHeader) ToV2() *session.XHeader {
 	return (*session.XHeader)(x)
 }

pkg/xheader_test.go

@@ -3,6 +3,7 @@ package pkg
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
 	"github.com/stretchr/testify/require"
 )
 
@@ -23,3 +24,19 @@ func TestXHeader(t *testing.T) {
 	require.Equal(t, key, xV2.GetKey())
 	require.Equal(t, val, xV2.GetValue())
 }
+
+func TestNewXHeaderFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *session.XHeader
+
+		require.Nil(t, NewXHeaderFromV2(x))
+	})
+}
+
+func TestXHeader_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *XHeader
+
+		require.Nil(t, x.ToV2())
+	})
+}