Remove support for the default null token on index fields

The default null token isn't supported anymore in Hibernate Search 6. Each field that takes advantage of the indexNullAs feature should define the null token explicitly.

The indexNullAs feature was deprecated in Infinispan 11: https://infinispan.org/docs/11.0.x/titles/upgrading/upgrading.html#entity_mappings

Also, Hibernate Search 6 offers an exist predicate, which is used in Ickle for IS NOT NULL conditions; so indexNullAs is no longer as important as before.