Make `filtersFromV2` return `nil` if passed
value is `nil`. Document field values of
instance constructed via `NewFilter`. Assert
the values in corresponding unit test.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
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>
