**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 constexpr (std::is_same_v<ToDataType, DataTypeDateTime64>)
        return true;
    else if constexpr (std::is_same_v<Name, NameToDateTime> || std::is_same_v<Name, NameParseDateTimeBestEffort>
        || std::is_same_v<Name, NameParseDateTimeBestEffortOrZero> || std::is_same_v<Name, NameParseDateTimeBestEffortOrNull>)
    {
        return (arguments.size() == 2 && isUnsignedInteger(arguments[1].type)) || arguments.size() == 3;
    }
    return false;
}
template <typename ToDataType, typename Name, typename MonotonicityImpl>
class FunctionConvert : public IFunction
{
public:
    using Monotonic = MonotonicityImpl;
    static constexpr auto name = Name::name;
    static constexpr bool to_decimal =
        std::is_same_v<Name, NameToDecimal32> || std::is_same_v<Name, NameToDecimal64>
         || std::is_same_v<Name, NameToDecimal128> || std::is_same_v<Name, NameToDecimal256>;
    static constexpr bool to_datetime64 = std::is_same_v<ToDataType, DataTypeDateTime64>;
    static constexpr bool to_string_or_fixed_string = std::is_same_v<ToDataType, DataTypeFixedString> ||
                                                      std::is_same_v<ToDataType, DataTypeString>;
    static constexpr bool to_date_or_datetime = std::is_same_v<ToDataType, DataTypeDate> ||
                                                std::is_same_v<ToDataType, DataTypeDate32> ||
                                                std::is_same_v<ToDataType, DataTypeDateTime>;
    static FunctionPtr create(ContextPtr) { return std::make_shared<FunctionConvert>(); }