zink: allow null descriptor set layouts

zink: allow null descriptor set layouts

I got confused while writing this somehow because of the null descriptor
feature, which enables drivers to consume a null descriptor, which has no
relation to a descriptor layout containing no descriptors

failing to accurately use zero descriptors can put layouts over the maximum
per-stage limits, which causes tests to crash

fixes (lavapipe):
KHR-GL46.shading_language_420pack.binding_uniform_block_array
KHR-GL46.multi_bind.dispatch_bind_buffers_base

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15067>