Fix error when parent is null in core-overlay

[jjaeggli]

When unit testing a component using paper-action-dialog, the test fails when opened is toggled on the element with the following errors:

core-overlay.html:458: Uncaught TypeError: Cannot read property 'removeChild' of null

and subsequently when the above error was addressed:

core-overlay-layer.html:92: Uncaught TypeError: Cannot read property 'removeChild' of null

these errors seem to correspond to the issue reported here:
dart-archive/paper-elements#83

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project, in which case you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[jjaeggli]

I signed it!

[googlebot]

CLAs look good, thanks!