Return transparent black when sampling null shader children

Bug: b/40045168
Bug: b/300305231
Change-Id: I35f051a34885d47c4b82eac30dcc86b34a92b9c7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/755636
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: John Reck <jreck@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed