benchmark: fixes csv parsing given no parameters

When a benchmark did not contain any parameters the csv configuration
filed would be "". In R this is by default parsed as NA, causing NA in
the printout too.

Fixes: https://github.com/nodejs/node/issues/9061
PR-URL: https://github.com/nodejs/node/pull/9064


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>