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.
No need to update the changelog here; we'll do it when we do a release (and organize it in a way that hopefully makes more sense than the individual commit bullets points).