**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 (to_nullable && WhichDataType(from_type).isStringOrFixedString())
                done = callOnIndexAndDataType<ToDataType>(from_type->getTypeId(), call, ConvertReturnNullOnErrorTag{});
            else
                done = callOnIndexAndDataType<ToDataType>(from_type->getTypeId(), call, ConvertDefaultBehaviorTag{});
        }
        if (!done)
        {
            
            if (std::is_same_v<ToDataType, DataTypeString>)
            {
                return ConvertImplGenericToString<ColumnString>::execute(arguments, result_type, input_rows_count);
            }
            else
                throw Exception("Illegal type " + arguments[0].type->getName() + " of argument of function " + getName(),
                    ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
        }
        return result_column;
    }
};
/** Function toTOrZero (where T is number of date or datetime type):
  *  try to convert from String to type T through parsing,
  *  if cannot parse, return default value instead of throwing exception.
  * Function toTOrNull will return Nullable type with NULL when cannot parse.
  * NOTE Also need to implement tryToUnixTimestamp with timezone.
  */
template <typename ToDataType, typename Name,
    ConvertFromStringExceptionMode exception_mode,
    ConvertFromStringParsingMode parsing_mode = ConvertFromStringParsingMode::Normal>
class FunctionConvertFromString : public IFunction