[skottie] Harden motion blur

1) don't attempt to attach the effect when the layer is null

2) sksg::onRender can receive a null context - handle it gracefully

TBR=

Change-Id: I4fae08b15d448849c7c99b17df6811ad31f190c8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/237276
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed