hash_value for std::nullptr_t is missing

[pdimov]

This is needed for

std::size_t boost::json::hash_value( value const& jv ) noexcept
{
    std::size_t seed = 0;

    boost::hash_combine( seed, jv.kind() == kind::int64? kind::uint64: jv.kind() );
    boost::json::visit( [&]( auto const& v ){ boost::hash_combine( seed, v ); }, jv );

    return seed;
}