build_plan: Don't run builds if no files found for input changes.

If no source path mapping found for a change it means it does not affect
CQ. In these cases do not run any builds.

BUG=chromium:1127243
TEST=./run_tests and local run

Change-Id: I7c79451c8b18a6c71528f1d0d1683a080c1668ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2406838
Commit-Queue: Navil Perez <navil@google.com>
Tested-by: Navil Perez <navil@google.com>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
3 files changed