- semantic.py: normalize both None and [] doc_types to null in the
metadata so a future `metadata->'doc_types' IS NULL` query counts the
all-types case consistently.
- test: use a fixed past date in test_occurred_at_roundtrip instead of a
future literal (deterministic, no "why this date" confusion).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>