**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 (checkAndGetDataType<DataTypeString>(from_type.get()))
        {
            return &ConvertImplGenericFromString<ColumnString>::execute;
        }
        return createWrapper<ToDataType>(from_type, to_type, requested_result_is_nullable);
    }
    WrapperType createUInt8ToBoolWrapper(const DataTypePtr from_type, const DataTypePtr to_type) const
    {
        return [from_type, to_type] (ColumnsWithTypeAndName & arguments, const DataTypePtr &, const ColumnNullable *, size_t ) -> ColumnPtr
        {
            
            
            
            auto res_column = to_type->createColumn();
            const auto & data_from = checkAndGetColumn<ColumnUInt8>(arguments[0].column.get())->getData();
            auto & data_to = assert_cast<ColumnUInt8 *>(res_column.get())->getData();
            data_to.resize(data_from.size());
            for (size_t i = 0; i != data_from.size(); ++i)
                data_to[i] = static_cast<bool>(data_from[i]);
            return res_column;
        };
    }
    static WrapperType createStringWrapper(const DataTypePtr & from_type)
    {
        FunctionPtr function = FunctionToString::create();
        return createFunctionAdaptor(function, from_type);
    }
    WrapperType createFixedStringWrapper(const DataTypePtr & from_type, const size_t N) const