**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 ret;
        if (callOnIndexAndDataType<void>(to_type->getTypeId(), make_default_wrapper))
            return ret;
        switch (to_type->getTypeId())
        {
            case TypeIndex::String:
                return createStringWrapper(from_type);
            case TypeIndex::FixedString:
                return createFixedStringWrapper(from_type, checkAndGetDataType<DataTypeFixedString>(to_type.get())->getN());
            case TypeIndex::Array:
                return createArrayWrapper(from_type, static_cast<const DataTypeArray &>(*to_type));
            case TypeIndex::Tuple:
                return createTupleWrapper(from_type, checkAndGetDataType<DataTypeTuple>(to_type.get()));
            case TypeIndex::Map:
                return createMapWrapper(from_type, checkAndGetDataType<DataTypeMap>(to_type.get()));
            case TypeIndex::Object:
                return createObjectWrapper(from_type, checkAndGetDataType<DataTypeObject>(to_type.get()));
            case TypeIndex::AggregateFunction:
                return createAggregateFunctionWrapper(from_type, checkAndGetDataType<DataTypeAggregateFunction>(to_type.get()));
            default:
                break;
        }
        if (cast_type == CastType::accurateOrNull)
            return createToNullableColumnWrapper();
        else
            throw Exception{"Conversion from " + from_type->getName() + " to " + to_type->getName() + " is not supported",
                ErrorCodes::CANNOT_CONVERT_TYPE};
    }
};
class MonotonicityHelper