don't produce empty SubRuns

Change-Id: Ib2d91afebbf6b7300a5820b25661a33b57996f42
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301304
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
3 files changed