**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+
{
        return [element_wrappers = getElementWrappers(from_kv_types, to_kv_types), from_kv_types, to_kv_types]
            (ColumnsWithTypeAndName & arguments, const DataTypePtr &, const ColumnNullable * nullable_source, size_t ) -> ColumnPtr
        {
            const auto * col = arguments.front().column.get();
            const auto & column_array = typeid_cast<const ColumnArray &>(*col);
            const auto & nested_data = typeid_cast<const ColumnTuple &>(column_array.getData());
            Columns converted_columns(2);
            for (size_t i = 0; i < 2; ++i)
            {
                ColumnsWithTypeAndName element = {{nested_data.getColumnPtr(i), from_kv_types[i], ""}};
                converted_columns[i] = element_wrappers[i](element, to_kv_types[i], nullable_source, (element[0].column)->size());
            }
            return ColumnMap::create(converted_columns[0], converted_columns[1], column_array.getOffsetsPtr());
        };
    }
    WrapperType createMapWrapper(const DataTypePtr & from_type_untyped, const DataTypeMap * to_type) const