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: d4c74aed7a (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>

src/amd/vulkan/radv_nir_to_llvm.c

@@ -2363,7 +2363,7 @@ handle_vs_outputs_post(struct radv_shader_context *ctx,
 		outinfo->export_prim_id = true;
 	}
 
-	if (export_layer_id) {
+	if (export_layer_id && layer_value) {
 		LLVMValueRef values[4];
 
 		values[0] = layer_value;