test: handle blank shells in test-os.js

The shell in /etc/passwd can be blank, in which case the user is given
the default shell. Handle this by only checking the shell contains a
path separator if the string isn't empty.

PR-URL: https://github.com/nodejs/node/pull/16287
Fixes: https://github.com/nodejs/node/issues/15684


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>