benchmark: allow zero when parsing http req/s

Without this, the http benchmarker would report "strange output" if
wrk or any other http client reported 0 requests per second, which is
a valid result.

PR-URL: https://github.com/nodejs/node/pull/10558


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>