Fix calls to deprecated Wuffs API

In the latest Wuffs' terminology, "null_foo" refers to an unusable foo
and "empty_foo" refers to a usable but zero-sized foo.

Bug: skia:8235
Change-Id: I7ac7128a0b370b2d32b5d35f13643d49c519eaca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243597
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
1 file changed