fix empty region iter rect

Region iterators return bounds but do not initialize
the result if Iterator has no region or region is
empty.

Add general tests for empty region iterators.

R=reed@google.com,halcanary@google.com

Docs-Preview: https://skia.org/?cl=149980
Bug: skia:8186
Change-Id: If4367d4c0eef7fc9c7bfd7d1436143e155b78309
Reviewed-on: https://skia-review.googlesource.com/149980
Commit-Queue: Cary Clark <caryclark@skia.org>
Reviewed-by: Hal Canary <halcanary@google.com>
2 files changed