change getBounds to return 0000 iff there are zero points

This is a contract change for SkPath::getBounds(), which formally was defined to return 0,0,0,0 for a 1-point path, regardless of the coordinates of that point. This seems wacky/inconsistent, and was causing other bugs (incorrect bounds) when this was unioned with other rects.

Does anyone remember why we defined it this way?

BUG=513799

Review URL: https://codereview.chromium.org/1261773002