Preserve null for empty relation SUM instead of implicit zero #271

Core program: teaql/teaql-conformance#236. The maintainer confirmed COUNT(empty)=0 and SUM(empty or all-null)=null; an explicit caller fallback may return zero. PortableSQLDataService.attachDynamicAggregations currently initializes every single-number aggregate to 0 regardless of its operator. This invents a SUM result for absent relation buckets. Scope: operator-aware missing-bucket materialization, nullable values retained through dynamic properties, real SQLite regressions and a generated Q/E example. Preserve COUNT behavior, real zero and per-statement TraceChain; do not change physical schema or hand-edit generated implementations. Verify the old implementation fails the same regression before qualifying the candidate. Local source and internal artifacts only; no public release authorized.