kernel: Fix length check for patch workflow

We want to skip the patch checks if there is an empty, but not nil, list
of patches. Update the length check accordingly.

BUG=None
TEST=presubmit

Change-Id: I34992817093e5b893d603b9a8827b27f3048ad9f
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/173666
Reviewed-by: Derek Taylor <ddtaylor@google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Tested-by: Kevin Berry <kpberry@google.com>