Detect empty clip when difference op clips everything

Change-Id: Ifbe0f3ae6e01d65f18351903da8aef63170ce6c7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294457
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed