tools: add cpplint rule for NULL usage

This commit is a suggestion for adding a rule for NULL usages in the
code base. This will currently report a number of errors which could be
ignored using // NOLINT (readability/null_usage)

PR-URL: https://github.com/nodejs/node/pull/17373


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>