main-custom: Fix default benchmark subTimes

A typo caused undefined values to leak into the geomean and break
the output parsing.

Bug: 358290763
Change-Id: I09ec15e094bdcb6981ca8659c24c9887bbf94491
Reviewed-on: https://chromium-review.googlesource.com/c/external/github.com/WebKit/JetStream/+/6663060
Reviewed-by: Daniel Lehmann <dlehmann@chromium.org>