SkSL: Remove "null" as a type and literal value.

Nullable fragment processors still exist, but they're handled
transparently by sample() within C++, so there's no need for .fp files
to ever do these tests manually.

Change-Id: Idf2bc58505207560553066c0126a2a036c5d970b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/347039
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
31 files changed