benchmark: ignore significance when using --runs 1

Because the standard deviation can't be calculated when there is only
one observation the R scripts raises an error. However it may still be
useful to run them for non-statistical purposes.

This changes the behaviour such when there is only one observation, the
values that depends on the standard deviation becomes Not Applicable
(NA).

Fixes: https://github.com/nodejs/node/issues/8288
PR-URL: https://github.com/nodejs/node/pull/8299


Reviewed-By: Anna Henningsen <anna@addaleax.net>