**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 createMapToObjectWrapper(*from_map, to_type->hasNullableSubcolumns());
        }
        else if (checkAndGetDataType<DataTypeString>(from_type.get()))
        {
            return [] (ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable * nullable_source, size_t input_rows_count)
            {
                auto res = ConvertImplGenericFromString<ColumnString>::execute(arguments, result_type, nullable_source, input_rows_count);
                auto & res_object = assert_cast<ColumnObject &>(res->assumeMutableRef());
                res_object.finalize();
                return res;
            };
        }
        else if (checkAndGetDataType<DataTypeObject>(from_type.get()))
        {
            return [is_nullable = to_type->hasNullableSubcolumns()] (ColumnsWithTypeAndName & arguments, const DataTypePtr & , const ColumnNullable * , size_t) -> ColumnPtr
            {
                auto & column_object = assert_cast<const ColumnObject &>(*arguments.front().column);
                auto res = ColumnObject::create(is_nullable);
                for (size_t i = 0; i < column_object.size(); i++)
                    res->insert(column_object[i]);
                res->finalize();
                return res;
            };
        }
        throw Exception(ErrorCodes::TYPE_MISMATCH,
            "Cast to Object can be performed only from flatten named Tuple, Map or String. Got: {}", from_type->getName());
    }
    template <typename FieldType>
    WrapperType createEnumWrapper(const DataTypePtr & from_type, const DataTypeEnum<FieldType> * to_type) const