resolveSort produced Sort.by(direction, "documentDate") with NATIVE null
handling, so Postgres surfaced undated (null meta_date) documents FIRST on
an ASC sort. Apply nullsLast() so undated rows order last for both ASC and
DESC, with a createdAt-asc tiebreaker for a stable total order when every
row is null-dated (the upcoming "Nur undatierte" filter).
Refs #668