Revert "Revert "SkSL now supports null child processors""

This reverts commit bce7d8627045ff5ca51784edf84caa88e87e0162.

Bug: skia:
Change-Id: I3ccaf47b3d4b9dcb9e049671117183a374526032
Reviewed-on: https://skia-review.googlesource.com/c/194185
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
26 files changed