Guard GrRadialGradientLayout::TestCreate against Empty shaders

With certain degenerate params (decal mode and a very small radius), we
could produce an SkShader that's an instance of SkEmptyShader, which
then failed to create an FP.

Bug: skia:8660 skia:8860 skia:8982
Change-Id: I47142572cd0c75d15c6ee1e66e3140786431371f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208221
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed