remove kReturnNullForEmpty_FinishFlag feature

It's unused, and doesn't appear to be correct, returning nullptr too
early when we should return a non-empty SkMiniPicture.

The mini-recorder path will return a zero-allocation SkEmptyPicture anyway.

Change-Id: I1be538049e731acfc2b0f8b4f30cafee434a91dc
Reviewed-on: https://skia-review.googlesource.com/12626
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed