test: check inspect array with empty string key

PR-URL: https://github.com/nodejs/node/pull/15258
Refs: https://github.com/nodejs/node/issues/15159


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>