undatedOnly(false) is a no-op (null predicate); undatedOnly(true) returns
documentDate IS NULL, matching the existing hasStatus null-as-no-op pattern.
Real-Postgres tests pin the load-bearing guarantees H2 cannot prove: ASC
NULLS-LAST ordering, BETWEEN excludes null-dated rows, and that undated=true
combined with a from/to range returns empty (the collision rule).
Refs #668