Handle null vertex buffers in Metal
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 <39df0a804564ccb6cf75f18db79653821f37c1c5@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia