radv: Avoid calling memcpy with null pointers

radv: Avoid calling memcpy with null pointers

Vulkan allows for these input pointers to be null when the respective
object count is zero. Calling memcpy with null pointers is undefined,
so they are guarded with a check for the legit use pattern now.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6728>