**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+
col_null_map_to = ColumnUInt8::create(size);
            vec_null_map_to = &col_null_map_to->getData();
        }
        const ColumnString::Chars * chars = nullptr;
        const IColumn::Offsets * offsets = nullptr;
        size_t fixed_string_size = 0;
        if constexpr (std::is_same_v<FromDataType, DataTypeString>)
        {
            chars = &col_from_string->getChars();
            offsets = &col_from_string->getOffsets();
        }
        else
        {
            chars = &col_from_fixed_string->getChars();
            fixed_string_size = col_from_fixed_string->getN();
        }
        size_t current_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            size_t next_offset = std::is_same_v<FromDataType, DataTypeString> ? (*offsets)[i] : (current_offset + fixed_string_size);
            size_t string_size = std::is_same_v<FromDataType, DataTypeString> ? next_offset - current_offset - 1 : fixed_string_size;
            ReadBufferFromMemory read_buffer(&(*chars)[current_offset], string_size);
            if constexpr (exception_mode == ConvertFromStringExceptionMode::Throw)
            {
                if constexpr (parsing_mode == ConvertFromStringParsingMode::BestEffort)
                {
                    if constexpr (to_datetime64)
                    {
                        DateTime64 res = 0;
                        parseDateTime64BestEffort(res, col_to->getScale(), read_buffer, *local_time_zone, *utc_time_zone);