**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 ((std::is_same_v<Name, NameToString> && !arguments.empty() && (isDateTime64(arguments[0].type) || isDateTime(arguments[0].type)))
            
            || std::is_same_v<Name, NameToUnixTimestamp>
            
            || std::is_same_v<ToDataType, DataTypeDate> 
            
            || std::is_same_v<ToDataType, DataTypeDate32>
            
            || std::is_same_v<ToDataType, DataTypeDateTime>
            
            || std::is_same_v<ToDataType, DataTypeDateTime64>)
        {
            optional_args.push_back({"timezone", &isString<IDataType>, &isColumnConst, "const String"});
        }
        validateFunctionArgumentTypes(*this, arguments, mandatory_args, optional_args);
        if constexpr (std::is_same_v<ToDataType, DataTypeInterval>)
        {
            return std::make_shared<DataTypeInterval>(Name::kind);
        }
        else if constexpr (to_decimal)
        {
            UInt64 scale = extractToDecimalScale(arguments[1]);
            if constexpr (std::is_same_v<Name, NameToDecimal32>)
                return createDecimalMaxPrecision<Decimal32>(scale);
            else if constexpr (std::is_same_v<Name, NameToDecimal64>)
                return createDecimalMaxPrecision<Decimal64>(scale);
            else if constexpr (std::is_same_v<Name, NameToDecimal128>)
                return createDecimalMaxPrecision<Decimal128>(scale);
            else if constexpr (std::is_same_v<Name, NameToDecimal256>)