fix: querySelector APIs support null/undefined (closes #104)

[p-ob]

To demonstrate this, you can see this in action if you go to the demo in a browser that doesn't natively implement popover, and in the console execute:

document.querySelector(null);

This causes some interop issues with other 3rd party libraries.

Expected behavior

document.querySelector does not throw an error, and returns null.

Actual behavior

Error thrown: Uncaught TypeError: can't access property "includes", selector is null

Firefox:

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[keithamus]

I wonder who's calling querySelector with null, but, defence in depth 😂

[p-ob]

I wonder who's calling querySelector with null, but, defence in depth 😂

We use a tool called Appcues, and I'm sure it's technically a bug that doesn't actually manifest in any user issues. But the web supports it so 🤷‍♂️.