fix(vue): respect keepContentsMounted if passed as attribute (#28167)

Issue number: resolves #28165

---------

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

The overlay implementation in Vue only checks for truthy
`keepContentsMounted` values. When setting this prop as an attribute,
the value of it is `''` which is falsy. As a result, content does not
get mounted.

One of Vue's ESLint rules states that this should be supported:
https://eslint.vuejs.org/rules/prefer-true-attribute-shorthand.html

Part of the issue may also be that Vue does not know the type of this
property and so it assume "any":

> The shorthand form is not always equivalent! If a prop accepts
multiple types, but Boolean is not the first one, a shorthand prop won't
pass true.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- The overlay wrapper now checks for `''` values. If
`keepContentsMounted === ''` then the inner contents will be mounted
because this means the prop is being set using the attribute shorthand.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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

Dev build: `7.3.5-dev.11694621267.1e5f63c2`

packages/vue/src/vue-component-lib/overlays.ts

@@ -177,7 +177,14 @@ export const defineOverlayContainer = <Props extends object>(name: string, defin
         return h(
           name,
           { ...restOfProps, ref: elementRef },
-          (isOpen.value || restOfProps.keepContentsMounted) ? renderChildren() : undefined
+
+          /**
+           * When binding keepContentsMounted as an attribute
+           * i.e. <ion-modal keep-contents-mounted></ion-modal>
+           * the value of the prop will be the empty string which is
+           * why we still call renderChildren() in that case.
+           */
+          (isOpen.value || restOfProps.keepContentsMounted || restOfProps.keepContentsMounted === '') ? renderChildren() : undefined
         )
       }
     });