**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+
{
        writeText(x, type->getScale(), wb, false);
        return ReturnType(true);
    }
};
template <typename FieldType, typename Name>
struct ConvertImpl<DataTypeEnum<FieldType>, DataTypeNumber<FieldType>, Name, ConvertDefaultBehaviorTag>
{
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t )
    {
        return arguments[0].column;
    }
};
static ColumnUInt8::MutablePtr copyNullMap(ColumnPtr col)
{
    ColumnUInt8::MutablePtr null_map = nullptr;
    if (const auto * col_null = checkAndGetColumn<ColumnNullable>(col.get()))
    {
        null_map = ColumnUInt8::create();
        null_map->insertRangeFrom(col_null->getNullMapColumn(), 0, col_null->size());
    }
    return null_map;
}
template <typename FromDataType, typename Name>
requires (!std::is_same_v<FromDataType, DataTypeString>)
struct ConvertImpl<FromDataType, DataTypeString, Name, ConvertDefaultBehaviorTag>
{
    using FromFieldType = typename FromDataType::FieldType;
    using ColVecType = ColumnVectorOrDecimal<FromFieldType>;
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t )