SkPDF: empty shader boxes bad

BUG=skia:6315
Change-Id: I48172b4deb4c55b08310e8e3d68089cfafeaa951
Reviewed-on: https://skia-review.googlesource.com/9156
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed