test: use t.error

[Uzlopak]

Detected by my citgm worfklow

I guess we now return null instead of undefined, so the tests fail. Fixing this.

Checklist

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