fix: correct expandEnv bug when passing in a new object in the data option

[serdnam]

This pull request fixes a bug I encountered while using env-schema, which is reproduced here: https://github.com/serdnam/env-schema-expand-bug

I have added a test that failed prior to making the needed change and passes with said change, all other tests pass.

This is my first time contributing to the Fastify org, let me know if there is anything that I missed!

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

[serdnam]

Thank you!