Revert "SkSL now supports null child processors"

This reverts commit 0e8dc4ce36f900ea428abd30ebcecbcc679d580c.

Reason for revert: ASAN complaints

Original change's description:
> 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>

TBR=bsalomon@google.com,ethannicholas@google.com,fmalita@chromium.org

Change-Id: Ibf739b3c6afd32d44f6b20705922618c29960d16
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/194020
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
26 files changed