**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+
{
    const auto * arg_type = named_column.type.get();
    bool ok = checkAndGetDataType<DataTypeUInt64>(arg_type)
        || checkAndGetDataType<DataTypeUInt32>(arg_type)
        || checkAndGetDataType<DataTypeUInt16>(arg_type)
        || checkAndGetDataType<DataTypeUInt8>(arg_type);
    if (!ok)
        throw Exception("Illegal type of toDecimal() scale " + named_column.type->getName(), ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
    Field field;
    named_column.column->get(0, field);
    return field.get<UInt32>();
}
struct NameToUnixTimestamp { static constexpr auto name = "toUnixTimestamp"; };
struct AccurateConvertStrategyAdditions
{
    UInt32 scale { 0 };
};
struct AccurateOrNullConvertStrategyAdditions
{
    UInt32 scale { 0 };
};
struct ConvertDefaultBehaviorTag {};
struct ConvertReturnNullOnErrorTag {};
/** Conversion of number types to each other, enums to numbers, dates and datetimes to numbers and back: done by straight assignment.