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

Document that `PlacementPolicy.ToV2` method return `nil`
when called on `nil`. Document that `PlacementPolicyFromV2`
function return `nil` when called on `nil`. Write corresponding
unit tests.

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

pkg/netmap/policy.go

@@ -13,11 +13,15 @@ func NewPlacementPolicy() *PlacementPolicy {
 }
 
 // NewPlacementPolicyFromV2 converts v2 PlacementPolicy to PlacementPolicy.
+//
+// Nil netmap.PlacementPolicy converts to nil.
 func NewPlacementPolicyFromV2(f *netmap.PlacementPolicy) *PlacementPolicy {
 	return (*PlacementPolicy)(f)
 }
 
 // ToV2 converts PlacementPolicy to v2 PlacementPolicy.
+//
+// Nil PlacementPolicy converts to nil.
 func (p *PlacementPolicy) ToV2() *netmap.PlacementPolicy {
 	return (*netmap.PlacementPolicy)(p)
 }