build: ignore empty folders in test-addons

Fixes: https://github.com/nodejs/node/issues/14843
PR-URL: https://github.com/nodejs/node/pull/16031


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>