Fix lack of undefined for DOMStringMap indexer

[Pinpickle]

Fixes: microsoft/TypeScript#13792

Currently, the DOMStringMap interface doesn't respect that elements in the map may be undefined. This makes it easy to result in type errors around its usage.

This PR adds a union with the undefined type to the elements in order to accurately reflect its possible runtime types.

[msftclas]

Hi @Pinpickle, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

In order for us to evaluate and accept your PR, we ask that you sign a contribution license agreement. It's all electronic and will take just minutes. I promise there's no faxing. https://cla.microsoft.com.

TTYL, MSBOT;

[msftgits]

Hi, I am closing and re-opening this PR to bump the CLA bot. Sorry for the inconvenience!

[msftclas]

Hi @Pinpickle, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!
You've already signed the contribution license agreement. Thanks!
We will now validate the agreement and then real humans will evaluate your PR.

TTYL, MSBOT;

[zhengbli]

👍