Handle null vertex buffers in Metal

When using instanced rendering, we sometimes bind a null vertex
buffer. Make sure not to crash during the deferred bind.

Change-Id: Ia0351222a220c2ac947de52e9d6eb76146dd64b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/408357
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed