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

Document that `Selector.ToV2` method return `nil`
when called on `nil`. Document that `NewSelectorFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.

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

pkg/netmap/selector.go

@@ -161,11 +161,15 @@ func NewSelector() *Selector {
 }
 
 // NewSelectorFromV2 converts v2 Selector to Selector.
+//
+// Nil netmap.Selector converts to nil.
 func NewSelectorFromV2(f *netmap.Selector) *Selector {
 	return (*Selector)(f)
 }
 
 // ToV2 converts Selector to v2 Selector.
+//
+// Nil Selector converts to nil.
 func (s *Selector) ToV2() *netmap.Selector {
 	return (*netmap.Selector)(s)
 }