**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+
data_to.resize(size * (strlen("YYYY-MM-DD") + 1));
            else if constexpr (std::is_same_v<FromDataType, DataTypeDateTime>)
                data_to.resize(size * (strlen("YYYY-MM-DD hh:mm:ss") + 1));
            else if constexpr (std::is_same_v<FromDataType, DataTypeDateTime64>)
                data_to.resize(size * (strlen("YYYY-MM-DD hh:mm:ss.") + col_from->getScale() + 1));
            else
                data_to.resize(size * 3);   
            offsets_to.resize(size);
            WriteBufferFromVector<ColumnString::Chars> write_buffer(data_to);
            if (null_map)
            {
                for (size_t i = 0; i < size; ++i)
                {
                    bool is_ok = FormatImpl<FromDataType>::template execute<bool>(vec_from[i], write_buffer, &type, time_zone);
                    null_map->getData()[i] |= !is_ok;
                    writeChar(0, write_buffer);
                    offsets_to[i] = write_buffer.count();
                }
            }
            else
            {
                for (size_t i = 0; i < size; ++i)
                {
                    FormatImpl<FromDataType>::template execute<void>(vec_from[i], write_buffer, &type, time_zone);
                    writeChar(0, write_buffer);
                    offsets_to[i] = write_buffer.count();
                }
            }
            write_buffer.finalize();
            if (null_map)