Fix compilation error with std::nullptr_t caster

[dean0x7d]

Fixes #839.

Note that void_caster::load() always returns false, even for None. This applies to std::nullptr_t and std::nullopt_t. Is there be a valid use case to make conversion from None work?

[wjakob]

This looks good to me, assuming that it passes the CI tests :)

[dean0x7d]

I enabled the None -> std::nullptr_t and None -> std::nullopt_t conversions to support std::variant<int, std::nullptr_t> from #839.

It doesn't seem to conflict with anything else. It does seem a bit weird, so I hope I'm not overlooking anything here.

[jagerman]

LGTM.