**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+
{
                            if (!WhichDataType(result_type).isFixedString())
                                throw Exception(
                                    ErrorCodes::TYPE_MISMATCH, "Wrong result type {}. Expected FixedString", result_type->getName());
                            const auto * null_map = column_nullable ? &column_nullable->getNullMapData() : nullptr;
                            if (cast_ipv4_ipv6_default_on_conversion_error_value)
                                return convertToIPv6<IPStringToNumExceptionMode::Default>(arguments[0].column, null_map);
                            else
                                return convertToIPv6<IPStringToNumExceptionMode::Throw>(arguments[0].column, null_map);
                        };
                        return true;
                    }
                    ret = &ConvertImplGenericFromString<typename FromDataType::ColumnType>::execute;
                    return true;
                }
            }
            if constexpr (WhichDataType(ToDataType::type_id).isStringOrFixedString())
            {
                if (from_type->getCustomSerialization())
                {
                    ret = [](ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable *, size_t input_rows_count) -> ColumnPtr
                    {
                        return ConvertImplGenericToString<typename ToDataType::ColumnType>::execute(arguments, result_type, input_rows_count);
                    };
                    return true;
                }
            }
            return false;
        };
        if (callOnTwoTypeIndexes(from_type->getTypeId(), to_type->getTypeId(), make_custom_serialization_wrapper))