test: use index.js if package.json "main" is empty

Verify that the module loader uses index.js when the "main" property of
package.json is the empty string.

Refs: https://github.com/nodejs/node/issues/32013

PR-URL: https://github.com/nodejs/node/pull/32040


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>