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>
26 files changed