**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+
const IColumn & column_from,
        IColumn & column_to,
        const ISerialization & serialization_from,
        size_t input_rows_count,
        const PaddedPODArray<UInt8> * null_map = nullptr,
        const IDataType * result_type = nullptr)
    {
        static_assert(std::is_same_v<StringColumnType, ColumnString> || std::is_same_v<StringColumnType, ColumnFixedString>,
                "Can be used only to parse from ColumnString or ColumnFixedString");
        if (const StringColumnType * col_from_string = checkAndGetColumn<StringColumnType>(&column_from))
        {
            column_to.reserve(input_rows_count);
            FormatSettings format_settings;
            for (size_t i = 0; i < input_rows_count; ++i)
            {
                if (null_map && (*null_map)[i])
                {
                    column_to.insertDefault();
                    continue;
                }
                const auto & val = col_from_string->getDataAt(i);
                ReadBufferFromMemory read_buffer(val.data, val.size);
                serialization_from.deserializeWholeText(column_to, read_buffer, format_settings);