[EuiIcon] Add an empty placeholder

The purpose of this task is to add an empty placeholder for missing or unknown `EuiIcon` types instead of showing the browser's broken-image fallback.

## Benchmark

Most design systems do **not** ship a special missing-icon picture. Closest peers to EUI (string `type` + runtime lookup) use empty + `console.error` (`console.warn` would get easily omitted in Kibana).

- **Font Awesome (SVG+JS)** - default `showMissingIcons: true`: animated `?` glyph. [docs](https://docs.fontawesome.com/apis/javascript/configuration)
- **Chakra UI v1/v2** - `` with no children → question-mark-in-circle. [docs](https://v2.chakra-ui.com/docs/components/icon)
- **Material Icons (font)** - unknown ligature → raw text / tofu. [FAQ](https://developers.google.com/fonts/faq) · [MUI](https://mui.com/material-ui/icons/)
- **Fluent UI v8** - unregistered `iconName` → `console.warn` + empty ``. [source](https://github.com/microsoft/fluentui/blob/master/packages/style-utilities/src/utilities/icons.ts) · [wiki](https://github.com/microsoft/fluentui/wiki/Using-icons)
- **Carbon Angular** - `IconNameNotFoundError` → `console.warn`, empty. [error](https://angular.carbondesignsystem.com/documentation/classes/IconNameNotFoundError.html) · [directive](https://angular.carbondesignsystem.com/documentation/directives/IconDirective.html)
- **Font Awesome React** — console error, no missing glyph. [issue](https://github.com/FortAwesome/react-fontawesome/issues/235)
- **MUI / Ant Design v4+ / Carbon React / Polaris / Spectrum / Primer Octicons / Atlassian / Cloudscape** - compile-time components; unknown names fail at TS/bundler time, no UI fallback. [MUI](https://mui.com/material-ui/icons/) · [Ant Design](https://ant.design/components/icon/) · [Polaris](https://polaris.shopify.com/components/images-and-icons/icon) · [Spectrum](https://react-spectrum.adobe.com/icons) · [Octicons](https://github.com/primer/octicons) · [Atlassian](https://atlassian.design/components/icon) · [Cloudscape](https://cloudscape.design/get-started/dev-guides/flexible-iconography/)
- **EUI today** - known icons while loading: blank `empty` SVG; unknown strings treated as URLs (``) → browser broken image.

- [ ] Unknown glyph doesn't result in the broken-image browser placeholder
- [ ] There is a runtime warning that warns about the missing icon