**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 ColumnNullable::create(std::move(col_to), std::move(null_map));
            return col_to;
        }
        else
            throw Exception("Illegal column " + arguments[0].column->getName()
                    + " of first argument of function " + Name::name,
                ErrorCodes::ILLEGAL_COLUMN);
    }
};
template <typename StringColumnType>
struct ConvertImplGenericToString
{
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t )
    {
        static_assert(std::is_same_v<StringColumnType, ColumnString> || std::is_same_v<StringColumnType, ColumnFixedString>,
                "Can be used only to serialize to ColumnString or ColumnFixedString");
        ColumnUInt8::MutablePtr null_map = copyNullMap(arguments[0].column);
        const auto & col_with_type_and_name = columnGetNested(arguments[0]);
        const IDataType & type = *col_with_type_and_name.type;
        const IColumn & col_from = *col_with_type_and_name.column;
        size_t size = col_from.size();
        auto col_to = removeNullable(result_type)->createColumn();
        {
            ColumnStringHelpers::WriteHelper write_helper(
                    assert_cast<StringColumnType &>(*col_to),
                    size);
            auto & write_buffer = write_helper.getWriteBuffer();
            FormatSettings format_settings;
            auto serialization = type.getDefaultSerialization();
            for (size_t i = 0; i < size; ++i)