[#302] pkg/netmap: Convert nil `Filter` to nil message

Make `Filter.ToV2` method return `nil`
when called on `nil`. Make `NewFilterFromV2`
function return `nil` when called on `nil`.
Write corresponding unit tests.

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

pkg/netmap/filter.go

@@ -153,11 +153,15 @@ func NewFilter() *Filter {
 }
 
 // NewFilterFromV2 converts v2 Filter to Filter.
+//
+// Nil netmap.Filter converts to nil.
 func NewFilterFromV2(f *netmap.Filter) *Filter {
 	return (*Filter)(f)
 }
 
 // ToV2 converts Filter to v2 Filter.
+//
+// Nil Filter converts to nil.
 func (f *Filter) ToV2() *netmap.Filter {
 	return (*netmap.Filter)(f)
 }

pkg/netmap/filter_test.go

@@ -213,6 +213,18 @@ func testFilter() *Filter {
 }
 
 func TestFilterFromV2(t *testing.T) {
+	t.Run("nil from V2", func(t *testing.T) {
+		var x *netmap.Filter
+
+		require.Nil(t, NewFilterFromV2(x))
+	})
+
+	t.Run("nil to V2", func(t *testing.T) {
+		var x *Filter
+
+		require.Nil(t, x.ToV2())
+	})
+
 	fV2 := new(netmap.Filter)
 	fV2.SetOp(netmap.GE)
 	fV2.SetName("name")