Cleanup in pictureshader
Cleanup in pictureshader

- fix a TODO to use shared decomposeScale
- return null rather than empty shader on degenerate input (more clear)

Landing this *before* trying to rebaseline chrome's images

Change-Id: I621eaff4dd3d958a1555c470578550483e9eb760
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352257
Reviewed-by: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Commit-Queue: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia