**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+
{
        const char * function_name = cast_name;
        return [function_name] (
            ColumnsWithTypeAndName & arguments, const DataTypePtr & res_type, const ColumnNullable * nullable_col, size_t )
        {
            const auto & first_col = arguments.front().column.get();
            const auto & result_type = typeid_cast<const EnumType &>(*res_type);
            const ColumnStringType * col = typeid_cast<const ColumnStringType *>(first_col);
            if (col && nullable_col && nullable_col->size() != col->size())
                throw Exception("ColumnNullable is not compatible with original", ErrorCodes::LOGICAL_ERROR);
            if (col)
            {
                const auto size = col->size();
                auto res = result_type.createColumn();
                auto & out_data = static_cast<typename EnumType::ColumnType &>(*res).getData();
                out_data.resize(size);
                auto default_enum_value = result_type.getValues().front().second;
                if (nullable_col)
                {
                    for (size_t i = 0; i < size; ++i)
                    {
                        if (!nullable_col->isNullAt(i))
                            out_data[i] = result_type.getValue(col->getDataAt(i));
                        else
                            out_data[i] = default_enum_value;
                    }