fix(clickhouse): preserve nulls in query results - #657
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A real ClickHouse differential replay exposed that the backend omitted nullable JSON fields and encoded TSV NULL as an empty string. Native ClickHouse returns explicit JSON nulls and the default
\NTSV marker, so successful DAG execution could still return a different result document.Enable explicit nulls in both Arrow JSON writers and emit the default TSV null marker before string escaping. A client override of
format_tsv_null_representationtakes the existing exact fallback path. Empty strings and literal\Nstrings remain distinct from SQL NULL.Validation: the regression covers JSON, JSONEachRow and TSV; the real Planner → installed DAG → backend HTTP differential replay that exposed the issue now passes against ClickHouse. Native TSV output was also checked directly. This fixes result encoding and makes no performance claim.