populate defaultValues in webidl dict. converter when passing null or undefined

[KhafraDev]

… undefined

The test explains the commit purpose better.

benchmarks/fetch/request-creation.mjs was consistently 10-15% faster. Not sure how @Uzlopak @tsctx, was this a fluke?

[KhafraDev]

I'm still confused how Request creation (with no RequestInit) is 10-15% faster.

[tsctx]

The constructor of Request has an empty object by default, so passing undefined to RequestInit will result in default value.
The performance increase is probably due to the removal of Object.hasOwn.

[KhafraDev]

I thought it could be Object.hasOwn, but the extra "overhead" from now having to loop over all of the converters made me think it would be slower. That's why I bothered benchmarking it in the first place.