radv: don't export NULL layer.

radv: don't export NULL layer.

We have some cases where in subpass we want the layer but having
it be 0 and loaded in the frag shader without the vertex shader
exporting it is fine.

So don't export the layer if we don't have a value to put in it.

Fixes: d4c74aed7a8 (radv/multiview: mark layer_input if we have input attachments.)
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>