[client-v2,jdbc-v2] `FORMAT null` support

### Description

See https://github.com/ClickHouse/clickhouse-java/issues/2715#issuecomment-5001979540

Start by reading the referenced issue comment and then locate the client-v2 and jdbc-v2 handling for FORMAT clauses. Determine the expected behavior for FORMAT null from that discussion, then verify both client paths with the relevant existing tests. Done means FORMAT null is accepted and behaves as agreed without regressing other formats.

索引モデルが issue の本文から書いたものです。