module: use undefined if no main

If the package.json file does not have a "main" entry, return undefined
rather than an empty string. This is to make more consistent behavior.
For example, when package.json is a directory, "main" is undefined
rather than an empty string.

PR-URL: https://github.com/nodejs/node/pull/18593


Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>