Small XJS Ast changes to match ast-types

Summary:
I found two cases where we are using `undefined` instead of `null`.
This doesn't match the general idea that `null` indiciates a missing node and
also doesn't match the ast-types spec here:
https://github.com/benjamn/ast-types/blob/master/def/fb-harmony.js

Test Plan:
node test/run.js
Made sure all fb js still parses
Ran jest tests
Merge pull request #27 from gabelevi/fb-harmony

Small XJS Ast changes to match ast-types