SkPath::getLastPt() to return optional

- clarifies to the caller that this can fail
- start updating internal call-sites

Change-Id: I1357dafcd473d1ba920e9044aa110a03a9a34a30
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1061316
Owners-Override: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Daniel Dilan <danieldilan@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>
6 files changed