fix: allow empty peer dependency metadata

[pengyuayn]

Summary

• Make PeerDependencyMeta.optional optional so PackageJSON accepts empty peer dependency metadata objects.
• Add a type-level regression test covering peerDependenciesMeta: { react: {} }.

Validation

• npx tap test/package-json.ts
• npm run eslint -- test/package-json.ts types/index.d.ts
• git diff --check

Notes

• npm test type-checks successfully but the full run currently fails because live npm registry fixture snapshots for old uuid versions changed their deprecated text upstream. That snapshot drift is unrelated to this change.