[#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)
 }