SkShaders::Blend/GPU: treat null inputs as input color

Matches CPU behavior.

Bug: skia:10019
Change-Id: I514bb996fdce8de2af5df008e185c48617a20a37
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/276008
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed