SkSL now supports null child processors Bug: skia: Change-Id: Icb3f2d57b393e3d40247aaad98c17344b507f0d7 Reviewed-on: https://skia-review.googlesource.com/c/193379 Reviewed-by: Brian Salomon <bsalomon@google.com> Reviewed-by: Florin Malita <fmalita@chromium.org> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>