Si Lin

Is possible jdbc do not check `null` just use `BinaryStreamUtils.writeNull(output);` to write `null` directly? I saw #935 and [insert_null_as_default](https://clickhouse.com/docs/en/operations/settings/settings/#insert_null_as_default) , i think it use to tell jdbc to deal with...

After i discuss with my workmate, We thought is not a bug. Because is a constraint，Column in DDL constraint to not nullable. When column set to `not null default xxx`...