**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+
src_converted_to_full_column = !removeNullable(arg.type)->equals(*removeNullable(res_type));
                    if (src_converted_to_full_column)
                        arg.column = arg.column->index(col_low_cardinality->getIndexes(), 0);
                    else
                        res_indexes = col_low_cardinality->getIndexesPtr();
                    tmp_rows_count = arg.column->size();
                }
                
                converted_column = wrapper(args, res_type, nullable_source, tmp_rows_count);
            }
            if (to_low_cardinality)
            {
                auto res_column = to_low_cardinality->createColumn();
                auto * col_low_cardinality = typeid_cast<ColumnLowCardinality *>(res_column.get());
                if (from_low_cardinality && !src_converted_to_full_column)
                {
                    col_low_cardinality->insertRangeFromDictionaryEncodedColumn(*converted_column, *res_indexes);
                }
                else
                    col_low_cardinality->insertRangeFromFullColumn(*converted_column, 0, converted_column->size());
                return res_column;
            }
            else if (!src_converted_to_full_column)
                return converted_column->index(*res_indexes, 0);
            else
                return converted_column;
        };
    }
    WrapperType prepareRemoveNullable(const DataTypePtr & from_type, const DataTypePtr & to_type, bool skip_not_null_check) const