Make GrFP::registerChild just return -1 for null children

Null children are becoming more common, so this centralizes the logic
that's currently handled via code-gen and manual checks.

Change-Id: Id8f4f4219b02690c2071ac29b36926dd50da23e9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301979
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
25 files changed