SQL: ClickHouse dialect NULL ordering, division and countIf follow DataFusion #655

Found in the #539 review.

Problem

With SqlDialect::ClickhouseSQL, the SQL lowering follows DataFusion's semantics in three places where ClickHouse's differ, so the plan can return different rows or values from what ClickHouse would:

• NULL ordering. ORDER BY x DESC lowers with nulls_first: true (DataFusion's default, nulls_max). ClickHouse puts NULLs last in both directions, so a top-k over a nullable column returns different rows.
• Division. bytes / 2 on integers is typed Int64 and divides as integers (5 → 2). ClickHouse's / always returns Float64 (2.5).
• countIf over no rows. It is rewritten to sum(CASE …), which is NULL over an empty input; ClickHouse returns 0.

The legacy SQL path has the same behaviour; #539 ported it unchanged.

Possible fix

Under ClickhouseSQL only:

• set datafusion.sql_parser.default_null_ordering = "nulls_last";
• cast both operands of / to Float64;
• wrap the countIf rewrite in coalesce(…, 0).

Each needs a ClickHouse-dialect test.