fix(index): return undefined on error instead of null

[Fdawgs]

closes #132

This is a breaking change so it'd be great to combine this with #72

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[gurgunday]

Oh I see, makes sense to me as null is valid JSON