code: protected <X> Specification<ENTITY> buildSpecification(Filter<X> filter, SingularAttribute<? super ENTITY, X>
          field) {
          return buildSpecification(filter, root -> root.get(field));
      }
docs: Helper function to return a specification for filtering on a single field, where equality, and null/non-null
  conditions are supported.
  
  @param filter the individual attribute filter coming from the frontend.
  @param field  the JPA static metamodel representing the field.
  @param <X>    The type of the attribute which is filtered.
  @return a Specification