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: #31372

PR-URL: #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>

benchmark/misc/getstringwidth.js

@@ -10,6 +10,8 @@ const bench = common.createBenchmark(main, {
 });
 
 function main({ n, type }) {
+  // Default value for testing purposes.
+  type = type || 'ascii';
   const { getStringWidth } = require('internal/readline/utils');
 
   const str = ({