Summary:
`nullptr` is typesafe. `0` and `NULL` are not. In the future, only `nullptr` will be allowed.
This diff helps us embrace the future _now_ in service of enabling `-Wzero-as-null-pointer-constant`.
Reviewed By: dmm-fb
Differential Revision: D55559752
fbshipit-source-id: 9f1edc836ded919022c4b53722f6f86208fecf8d