In some cases viper doesn't interpret `section.value` as a subsection with
`section` name, but value is value still can be accessed through full
pathname.
Fix `Config.Sub` method implementation in order to always interpret
configuration like described above as a subsection. From now method never
returns nil, therefore an additional check has been removed from the `Value`
method.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
1aa88159ca