Use std::nullopt instead of {} in SubRunContainer

Change-Id: I31ffecf0634b4f31751671f50e04ee7b2af5dbdb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/551893
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed