Use brace-initialization for folly::none

Summary:
[Folly] Use brace-initialization for `folly::none`.

Will help with future work.

Reviewed By: shixiao

Differential Revision: D9039671

fbshipit-source-id: fc34041bc040332540ed3e9dfd7ab452e82ca47f

folly/Optional.h

@@ -79,7 +79,7 @@ struct OptionalPromiseReturn;
 
 typedef int detail::NoneHelper::*None;
 
-const None none = nullptr;
+const None none = {};
 
 class FOLLY_EXPORT OptionalEmptyException : public std::runtime_error {
  public: