Treat `[undefined]` as valid `PropTypes.renderable`

- #1647: React no longer issues a warning when passing in `[undefined]` as `React.PropTypes.renderable`, hence treating it the same as `[null]`.

#### Notes
- Tests pass.
- Lint reports many warnings but not due to newly added code.
- Previously signed CLA.
:+1: Removed `CHANGELOG` entry.