Added Support of Nullable Types in Mysql Table Function

Why it matters

This feature enables users to correctly handle and query MySQL tables that contain Nullable columns using the mysql table function in ClickHouse. It solves the problem of data type incompatibility and improves data integration by preserving nullability when accessing MySQL data.

How to use it

Use the mysql table function as usual to query MySQL tables with nullable columns. The function now automatically supports and returns columns with Nullable types without additional configuration. Example usage:

SELECT * FROM mysql('host:port', 'database', 'table', 'user', 'password')