**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+
Additions additions [[maybe_unused]] = Additions())
    {
        return arguments[0].column;
    }
};
template <typename Name>
struct ConvertImpl<DataTypeUInt8, DataTypeUInt8, Name, ConvertDefaultBehaviorTag>
{
    template <typename Additions = void *>
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t ,
                             Additions additions [[maybe_unused]] = Additions())
    {
        return arguments[0].column;
    }
};
/** Conversion from FixedString to String.
  * Cutting sequences of zero bytes from end of strings.
  */
template <typename Name>
struct ConvertImpl<DataTypeFixedString, DataTypeString, Name, ConvertDefaultBehaviorTag>
{
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t )
    {
        if (const ColumnFixedString * col_from = checkAndGetColumn<ColumnFixedString>(arguments[0].column.get()))
        {
            auto col_to = ColumnString::create();
            const ColumnFixedString::Chars & data_from = col_from->getChars();
            ColumnString::Chars & data_to = col_to->getChars();
            ColumnString::Offsets & offsets_to = col_to->getOffsets();