**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+
{
        ColumnUInt8::MutablePtr null_map = copyNullMap(arguments[0].column);
        const auto & col_with_type_and_name =  columnGetNested(arguments[0]);
        const auto & type = static_cast<const FromDataType &>(*col_with_type_and_name.type);
        const DateLUTImpl * time_zone = nullptr;
        
        if constexpr (std::is_same_v<FromDataType, DataTypeDateTime> || std::is_same_v<FromDataType, DataTypeDateTime64>)
        {
            auto non_null_args = createBlockWithNestedColumns(arguments);
            time_zone = &extractTimeZoneFromFunctionArguments(non_null_args, 1, 0);
        }
        if (const auto col_from = checkAndGetColumn<ColVecType>(col_with_type_and_name.column.get()))
        {
            auto col_to = ColumnString::create();
            const typename ColVecType::Container & vec_from = col_from->getData();
            ColumnString::Chars & data_to = col_to->getChars();
            ColumnString::Offsets & offsets_to = col_to->getOffsets();
            size_t size = vec_from.size();
            if constexpr (std::is_same_v<FromDataType, DataTypeDate>)
                data_to.resize(size * (strlen("YYYY-MM-DD") + 1));
            else if constexpr (std::is_same_v<FromDataType, DataTypeDate32>)