Enable nullish coalescing in flow config (#20516)

Summary:
Follow up to a9792ac4c8ee04f9832a99e95e1afebbb568d230. I assume it's enabled for fb flow config so we can avoid future breakages if someone decide to use this syntax.
Pull Request resolved: https://github.com/facebook/react-native/pull/20516

Differential Revision: D9169244

Pulled By: TheSavior

fbshipit-source-id: 3736d2aed65a0cb4392c175f7d652594cf6a1518