esm: empty ext from pkg type/main doesnt affect format

This ensures files with unknown extensions like foo.bar are not
loaded as CJS/ESM when imported as a main entry point and makes
sure that those files would maintain the same format even if loaded
after the main entrypoint.

PR-URL: https://github.com/nodejs/node/pull/31021


Reviewed-By: Guy Bedford <guybedford@gmail.com>