lib: simplify Module._resolveLookupPaths

This commit consists of two changes:

* Avoids returning request/id *just* for the debug() output
* Returns `null` instead of an empty array for the list of paths

PR-URL: https://github.com/nodejs/node/pull/10789


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>