Add a setting `insert_select_null_as_default`

**Use case**
```
CREATE TABLE default_test (s String DEFAULT 'Hello') ENGINE = Memory;

-- works, see the `input_format_null_as_default` setting.
INSERT INTO default_test VALUES ('World'), (NULL);

-- does not work but I want it to work:
INSERT INTO default_test SELECT 'World' UNION ALL SELECT NULL;
```
