pw_presubmit: Skip copyright check on empty files
pw_presubmit: Skip copyright check on empty files

Change-Id: Id6cc767df9e5f3648dff5d56c71890523b5f336c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/15820
Reviewed-by: Joe Ethier <f5eb8cb2f5310c7fa9df6f16eabccb4436f47999@google.com>
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed