**Describe what's wrong**

```sql
SELECT toTypeName(s), toString(s) FROM
(
 SELECT toFixedString('',32) AS s 
 UNION ALL
 SELECT null AS s 
)
```

this query works in all versions from 19.17 to 21.8, but stop works after 21.9+ 

https://fiddle.clickhouse.com/7bd65b36-a6e6-470a-8bdc-4c98a33ff453
21.8 works

https://fiddle.clickhouse.com/9e40a424-4027-4c7f-bd41-b20935ae9129
21.9 doesn't work
```
Received exception from server (version 21.9.6):
Code: 44. DB::Exception: Received from localhost:9000. DB::Exception: Illegal column Nullable(FixedString(32)) of first argument of function toString. (ILLEGAL_COLUMN)
```

this query works in all versions

```sql
SELECT toTypeName(s), toString(s) FROM
(
 SELECT '' AS s 
 UNION ALL
 SELECT null AS s 
)
```
https://fiddle.clickhouse.com/4f810f03-1644-4eb7-b696-0341b1bbdbc4


**Does it reproduce on recent release?**
Yes, reproduce in all releases 21.9+
for (auto & type : to_types)
        {
            if (isTuple(type) || isNested(type))
                throw Exception(ErrorCodes::TYPE_MISMATCH,
                    "Cast to Object can be performed only from flatten Named Tuple. Got: {}",
                    from_tuple.getName());
            type = recursiveRemoveLowCardinality(type);
        }
        return [element_wrappers = getElementWrappers(from_types, to_types),
            has_nullable_subcolumns, from_types, to_types, paths]
            (ColumnsWithTypeAndName & arguments, const DataTypePtr &, const ColumnNullable * nullable_source, size_t input_rows_count)
        {
            size_t tuple_size = to_types.size();
            auto flattened_column = flattenTuple(arguments.front().column);
            const auto & column_tuple = assert_cast<const ColumnTuple &>(*flattened_column);
            if (tuple_size != column_tuple.getColumns().size())
                throw Exception(ErrorCodes::TYPE_MISMATCH,
                    "Expected tuple with {} subcolumn, but got {} subcolumns",
                    tuple_size, column_tuple.getColumns().size());
            auto res = ColumnObject::create(has_nullable_subcolumns);
            for (size_t i = 0; i < tuple_size; ++i)
            {
                ColumnsWithTypeAndName element = {{column_tuple.getColumns()[i], from_types[i], "" }};
                auto converted_column = element_wrappers[i](element, to_types[i], nullable_source, input_rows_count);
                res->addSubcolumn(paths[i], converted_column->assumeMutable());
            }
            return res;
        };
    }
    WrapperType createMapToObjectWrapper(const DataTypeMap & from_map, bool has_nullable_subcolumns) const