Resource is optional in `napi_async_init`

[legendecas]

Maybe we should also think about adapting napi_async_init as it states that the resource is optional. As Node is meanwhile built to require an async resource it seems to be strange that we create an object on the fly if user doesn't provide one.
Removing the null check in napi_async_init would be no issue from ABI point of view but breaking from behavior. Not sure if this should be included in this PR.

Originally posted by @Flarna in #32930

[mhdawson]

@legendecas, @Flarna this is what I'd suggest we do:

#33181

I don't think we should change the implementation as it would be breaking which we can't do for N-API but updating the doc does make sense.

[mhdawson]

I've left as draft until I get feedback from @legendecas and @Flarna that this makes sense to them as well.