[graphite] Fix SkEmpty (take 2)

Bug: chromium:1517553
Change-Id: I19b94f52b2f2d32163b531628463a6a358d46ed7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/808218
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed