Make GrMesh compatible with instanced draws that have no attribs Bug: skia: Change-Id: Ia366e20608670df34ab2db3966f5beaa23242592 Reviewed-on: https://skia-review.googlesource.com/c/196162 Reviewed-by: Brian Salomon <bsalomon@google.com> Commit-Queue: Chris Dalton <csmartdalton@google.com>