Content of SVG elements is ignored in determining if the card is unique

> It seems that Anki (I’m on Version ⁨23.12.1 (1a1d4d54)⁩, Python 3.9.15 Qt 6.6.1 PyQt 6.6.1), SVG elements are ignored for determining if the card is empty or unique.
> To reproduce, try to create a card with Front
>

This is happening because we strip HTML from the field prior to determining if it's empty, as invisible br/b/etc tags would otherwise cause a visually-empty field to be accepted.

Originally reported on https://forums.ankiweb.net/t/content-of-svg-elements-is-ignored-in-determining-if-the-card-is-unique/51915

Start by locating the logic that strips HTML before checking whether a field is empty or a card is unique. Reproduce the issue with the provided SVG and verify that its content is considered while visually empty fields remain rejected; the issue provides no file or test path.

Written by the indexing model from the issue text.