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>

src/gpu/mtl/GrMtlOpsRenderPass.mm

@@ -416,6 +416,10 @@ void GrMtlOpsRenderPass::setVertexBuffer(id<MTLRenderCommandEncoder> encoder,
                                          const GrBuffer* buffer,
                                          size_t vertexOffset,
                                          size_t inputBufferIndex) {
+    if (!buffer) {
+        return;
+    }
+
     constexpr static int kFirstBufferBindingIdx = GrMtlUniformHandler::kLastUniformBinding + 1;
     int index = inputBufferIndex + kFirstBufferBindingIdx;
     SkASSERT(index < 4);