**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+
return createDecimalMaxPrecision<Decimal256>(scale);
            throw Exception("Unexpected branch in code of conversion function: it is a bug.", ErrorCodes::LOGICAL_ERROR);
        }
        else
        {
            
            UInt8 timezone_arg_position = 1;
            UInt32 scale [[maybe_unused]] = DataTypeDateTime64::default_scale;
            
            if (isDateTime64<Name, ToDataType>(arguments))
            {
                timezone_arg_position += 1;
                scale = static_cast<UInt32>(arguments[1].column->get64(0));
                if (to_datetime64 || scale != 0) 
                    return std::make_shared<DataTypeDateTime64>(scale,
                        extractTimeZoneNameFromFunctionArguments(arguments, timezone_arg_position, 0));
                return std::make_shared<DataTypeDateTime>(extractTimeZoneNameFromFunctionArguments(arguments, timezone_arg_position, 0));
            }
            if constexpr (std::is_same_v<ToDataType, DataTypeDateTime>)
                return std::make_shared<DataTypeDateTime>(extractTimeZoneNameFromFunctionArguments(arguments, timezone_arg_position, 0));
            else if constexpr (std::is_same_v<ToDataType, DataTypeDateTime64>)
                throw Exception("Unexpected branch in code of conversion function: it is a bug.", ErrorCodes::LOGICAL_ERROR);
            else
                return std::make_shared<ToDataType>();
        }
    }
    
    
    
    bool useDefaultImplementationForNulls() const override