Add guard to normalizeKeyName() for undefined keys + changeset

[Copilot]

🎯 Changes

event.key can be undefined in rare cases (browser extensions synthesizing key events, accessibility tools, certain OS/browser combinations). normalizeKeyName() called key.toUpperCase() without guarding, causing:

TypeError: Cannot read properties of undefined (reading 'toUpperCase')

• constants.ts: Early-return '' when key is falsy in normalizeKeyName()
• parse.test.ts: Tests for '' and undefined (coerced) inputs
• .changeset/guard-normalize-key-name-undefined.md: Patch changeset for @tanstack/hotkeys

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).