[EuiIcon] Add an empty placeholder

[weronikaolejniczak]

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
• Chakra UI v1/v2 - <Icon /> with no children → question-mark-in-circle. docs
• Material Icons (font) - unknown ligature → raw text / tofu. FAQ · MUI
• Fluent UI v8 - unregistered iconName → console.warn + empty <i>. source · wiki
• Carbon Angular - IconNameNotFoundError → console.warn, empty. error · directive
• Font Awesome React — console error, no missing glyph. issue
• 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 · Ant Design · Polaris · Spectrum · Octicons · Atlassian · Cloudscape
• EUI today - known icons while loading: blank empty SVG; unknown strings treated as URLs (<img src>) → browser broken image.

Takeaway: a dedicated missing glyph is Font Awesome's pattern, not the React DS default. Empty placeholder matches Fluent and Carbon Angular.

Acceptance criteria

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