message: MINOR: Fix coding style to mentioned undefined
diff: @@ -162,6 +162,7 @@ If an interface has data member, it's no longer abstract. Derived classes can't
  - Create private `setupXYZ` methods, if applicable.
  - Instead of `setupXYZ` methods, it might be possible to create a subclass of the object and initialize the members there.
  - Consider applying [Carmack's "good judgement"](http://number-none.com/blow/john_carmack_on_inlined_code.html) by breaking this rule ONLY if it is necessary.
  +1. Use `undefined`, don't use `null`. Inspired from: https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines#null-and-undefined
  ## Formatting
  