As documented, conjunction with zero conjuncts is true. Such a predicate can be created as follows:
CriteriaBuilder cb ...
Predicate conjunction = cb.conjunction();
When opposite is needed, that is task for disjunction (with zero disjuncts it is false):
CriteriaBuilder cb ...
Predicate disjunction = cb.disjunction();
Disjunction without disjunct typically generates something like 1=0 to SQL query.