anv: Always emit at least one vertex element

anv: Always emit at least one vertex element

This seems to make the simulator happier.  The early return wasn't
really protecting anything and the code that follows will happily
initialize the dummy element to STORE_0 and emit it.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>