Support NULL as Default for Nested Types in Input Formats

Why it matters

This feature addresses the limitation where nested data types could not default to NULL during data ingestion. It allows users to handle incomplete or missing nested data more gracefully by automatically assigning NULL as the default, improving data consistency and simplifying ETL processes.

How to use it

When inserting data using supported input formats, users can now specify NULL values for nested columns of type Array, Tuple, or Map, and ClickHouse will correctly interpret and store them as default NULL values without additional configuration.