**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+
}
        bool skip_not_null_check = false;
        if (from_low_cardinality && from_nested->isNullable() && !to_nested->isNullable())
            
            skip_not_null_check = true;
        auto wrapper = prepareRemoveNullable(from_nested, to_nested, skip_not_null_check);
        if (!from_low_cardinality && !to_low_cardinality)
            return wrapper;
        return [wrapper, from_low_cardinality, to_low_cardinality, skip_not_null_check]
                (ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable * nullable_source, size_t input_rows_count) -> ColumnPtr
        {
            ColumnsWithTypeAndName args = {arguments[0]};
            auto & arg = args.front();
            auto res_type = result_type;
            ColumnPtr converted_column;
            ColumnPtr res_indexes;
            
            bool src_converted_to_full_column = false;
            {
                auto tmp_rows_count = input_rows_count;
                if (to_low_cardinality)
                    res_type = to_low_cardinality->getDictionaryType();
                if (from_low_cardinality)
                {
                    const auto * col_low_cardinality = typeid_cast<const ColumnLowCardinality *>(arguments[0].column.get());
                    if (skip_not_null_check && col_low_cardinality->containsNull())
                        throw Exception{"Cannot convert NULL value to non-Nullable type",
                                        ErrorCodes::CANNOT_INSERT_NULL_IN_ORDINARY_COLUMN};
                    arg.column = col_low_cardinality->getDictionary().getNestedColumn();
                    arg.type = from_low_cardinality->getDictionaryType();