**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+
result_column = ConvertImpl<LeftDataType, RightDataType, Name, SpecialTag>::execute(arguments, result_type, input_rows_count);
            }
            return true;
        };
        if (isDateTime64<Name, ToDataType>(arguments))
        {
            
            const ColumnWithTypeAndName & scale_column = arguments[1];
            UInt32 scale = extractToDecimalScale(scale_column);
            if (to_datetime64 || scale != 0) 
            {
                if (!callOnIndexAndDataType<DataTypeDateTime64>(from_type->getTypeId(), call, ConvertDefaultBehaviorTag{}))
                    throw Exception("Illegal type " + arguments[0].type->getName() + " of argument of function " + getName(),
                                    ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
                return result_column;
            }
        }
        if constexpr (std::is_same_v<ToDataType, DataTypeString>)
        {
            if (from_type->getCustomSerialization())
                return ConvertImplGenericToString<ColumnString>::execute(arguments, result_type, input_rows_count);
        }
        bool done;
        if constexpr (to_string_or_fixed_string)
        {
            done = callOnIndexAndDataType<ToDataType>(from_type->getTypeId(), call, ConvertDefaultBehaviorTag{});
        }
        else
        {