[skottie] Handle empty lines in Shaper

SkShaper doesn't seem to like empty inputs.

TBR=
Change-Id: Iace3eef389051dcce229ed4ac6897344770d19c3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/224190
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Auto-Submit: Florin Malita <fmalita@chromium.org>
1 file changed