fix: ensure `totype` always returns stringified null/undefined when null/undefined is passed (#30383)

The `prev()` function doesn't take nodes other than elements into account. Also we could simplify things a lot using the `previousElementSibling` property. This property isn't fully supported in IE, it only works for elements, but since the `element` variable is an element, we can safely use it here.

I've also added an additional test.

I don't think we had this issue in v4, since we relied on jQuery back then.

Ref. https://developer.mozilla.org/en-US/docs/Web/API/NonDocumentTypeChildNode/nextElementSibling

"without instantiate it" should be "without instantiating it"

* Close modal with keyboard=true & backdrop=static

swap jQuery's `trigger(...)` with the more verbose native `dispatchEvent(...)`, as the former may not always behave/bubble correctly (observed while trying to write unit tests for keyboard handling of ARIA tab navigation), which may lead to this test passing even though it fails in real usage.

Add unit test from #28912 to v5

Just firing the focus event isn't enough ... need to actually call the proper `focus()` method for it to actually do it...