folly: avoid compile warning/failure due to lvalue-to-rvalue conversion

folly: avoid compile warning/failure due to lvalue-to-rvalue conversion

Summary:
With gcc 7.2 we get the warning:
  folly/io/async/DelayedDestructionBase.h:252:20:
  error: parameter ‘right’ set but not used [-Werror=unused-but-set-parameter]
     std::nullptr_t right) {
                    ^~~~~
I presume this is due to the implicit conversion, hence the named parameter is
never assigned.  Instead we use an explicit nullptr.

Reviewed By: yfeldblum

Differential Revision: D6279302

fbshipit-source-id: ed449601b0410c178777f20e82ed09d9097bd024