fix(popover): viewport can be scrolled if no content present (#29215)

Issue number: resolves #29211

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## What is the current behavior?
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In https://github.com/ionic-team/ionic-framework/pull/28861 I fixed a
bug that caused `.popover-viewport` to have `overflow: hidden`. In
reality, this code should have always applied but due to an incorrect
selector it never did.

As it turns out in
https://github.com/ionic-team/ionic-framework/issues/29211, some
developers were relying on the broken behavior to build their
applications. In particular, developers were using `ion-popover` without
an `ion-content`. The linked change made it so that using popovers
without `ion-content` were not scrollable.

After talking with @mapsandapps we think it makes sense to officially
support this behavior. We support using [modals without
`ion-content`](https://ionicframework.com/docs/api/modal#custom-dialogs),
and we could not think of a reason to not support the same use case for
popover.

## What is the new behavior?
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this PR. -->

- If the `.popover-viewport` element has a child content then
`.popover-viewport `will not be scrollable.
- If the `.popover-viewport` element does not have a child content then
`.popover-viewport` will be scrollable.

I implemented this behavior using progressive enhancement via `:has`.
The [`:has` pseudo-class](https://caniuse.com/?search=%3Ahas) has
cross-browser support. Ionic v7 supports some versions of browsers that
do not have `:has` support. As a result, we fall back to the existing
behavior in this environment. Developers are able to get this behavior
on older browsers by explicitly setting `overflow: auto` on
`.popover-viewport`.

Fortunately, we will be dropping support for several of the older
browsers versions in Ionic v8, so the need to do the manual opt-in
should be less frequent as time goes on.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

Dev build: `7.8.2-dev.11711383079.118d48a5`

Testing:

1. Open https://codepen.io/liamdebeasi/pen/JjVJrZQ?editors=1100 (this
has a dev build installed)
2. Click each button to open a popover.
3. Verify that each popover can be scrolled.

I could not find a great way to automate this test, but let me know if
anyone has ideas!