[card] Text node not shown when passed into default slot

[web-padawan]

Description

Unlike other components e.g. vaadin-badge, in vaadin-card the approach for _c attribute assumes an element, so the attribute isn't set if the content is a text node (e.g. Text component in Flow).

web-components/packages/card/src/vaadin-card.js

Line 160 in 37909af

this.toggleAttribute('_c', this.querySelector(':scope > :not([slot])'));

This is problematic especially for add(String text) method in Java - see vaadin/docs#5247

Expected outcome

I would expect the _c attribute to be set if there is a non-empty text node, using e.g. SlotController.

Minimal reproducible example

<vaadin-card>
 This text should be visible!
</vaadin-card>

Steps to reproduce

Add the snippet to the dev page and observe an empty card.

Environment

Vaadin version(s): 24.x, 25.x

Browsers

Issue is not browser related