Don't assert on empty GrGLPaths

Ensure asserts aren't fired when creating a GrGLPath path from an
empty SkPath. An empty path is valid and has practical use, for
example, as a template path for certain nvpr calls.

BUG=skia:2891
R=bsalomon@google.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/522883002