Document that `SplitID.ToV2` method return `nil`
when called on `nil`. Document that `NewSplitIDFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
Document that `NewTombstoneFromV2` function return
`nil` when called on `nil`. Write corresponding
unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
