allow conversion from nullptr to folly::Optional<bool>
allow conversion from nullptr to folly::Optional<bool>

Summary:
Disabling the nullptr conversion to non-pointer folly::Optional<T> has
baked long enough. Re-enabled it with the same behavior that
std::optional has.

Reviewed By: Orvid

Differential Revision: D15151503

fbshipit-source-id: d170f3f0e6bf3bb4461f17bdb6f8078b5a883615

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly