benchmark: add default type in getstringwidth.js

This fixes a benchmark test that was recently broken by a breaking
change on the master branch.

Fixes: https://github.com/nodejs/node/issues/31372

PR-URL: https://github.com/nodejs/node/pull/31377


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>