Enable nullish coalescing in flow config (#20516)

Summary:
Follow up to a9792ac. 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: #20516

Differential Revision: D9169244

Pulled By: TheSavior

fbshipit-source-id: 3736d2aed65a0cb4392c175f7d652594cf6a1518

.flowconfig

@@ -41,6 +41,7 @@ flow-github/
 emoji=true
 
 esproposal.optional_chaining=enable
+esproposal.nullish_coalescing=enable
 
 module.system=haste
 module.system.haste.use_name_reducers=true