Ensure alt attribute in custom icon img elements

[MichaelMarcialis]

Summary

This PR fixes the img element in EuiIcon components using custom SVGs to have an alt attribute with an empty string, rather than no alt attribute at all. This ensures validation while also indicating to screen readers to not read icons that are for decoration purposes only.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation examples
- [ ] Added or updated jest tests
- [ ] Checked for breaking changes and labeled appropriately

• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3245/

[myasonik]

I'm on the fence about approving this.

On one hand, this does work... On the other hand, I don't know if I trust devs to remember to pass in a value for title even if the icon has meaning.

The way it currently stands, pages will fail automated testing when/if that's the case and the images can be triaged. (Assuming that devs are doing automated testing...) That can be annoying but should be a one time cost. For Kibana as an example, as the automated testing gets broader and broader it should also stop these things from ever even being merged in the first place.

With this change, all images will appear to pass automated testing but false negatives are incredibly difficult to spot...

Can someone convince me one way or the other?

[miukimiu]

When the icons are rendered as SVGs if no title is provided they get an aria-hidden=true. So we assume it's for decoration purposes only.

We also have this EuiCallOut on top of the page to ensure devs pass a title:

For me, it makes sense the alt attribute with an empty string for icons rendered as img. So they will behave as decorative images same way the rendered SVGs are.

I think it will always be difficult to track false negatives. So we have to assume devs are going to follow the best practices we recommend in the docs.

But @myasonik I think you're the right person to make a decision here.

[myasonik]

@miukimiu You convinced me by saying this would bring it inline with how svgs are currently handled. At least it'll be consistent 😅

🚀

[miukimiu]

LGTM! 🎉