Refer to nullptr not NULL

Refer to nullptr not NULL

Summary:
Folly is a C++ library not a C library, and (almost) universally uses `nullptr` everywhere, so refer to `nullptr` rather than `NULL`.
This also fixes the 1 place in our actual code where we were using `NULL` rather than `nullptr`.

Reviewed By: yfeldblum

Differential Revision: D5204766

fbshipit-source-id: 2a5d5011a28d7d5dd48789f60643a656f51b9732