**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+
#include <DataTypes/DataTypeUUID.h>
#include <DataTypes/DataTypeInterval.h>
#include <DataTypes/DataTypeAggregateFunction.h>
#include <DataTypes/DataTypeObject.h>
#include <DataTypes/ObjectUtils.h>
#include <DataTypes/DataTypeNested.h>
#include <DataTypes/Serializations/SerializationDecimal.h>
#include <Formats/FormatSettings.h>
#include <Columns/ColumnString.h>
#include <Columns/ColumnFixedString.h>
#include <Columns/ColumnConst.h>
#include <Columns/ColumnArray.h>
#include <Columns/ColumnNullable.h>
#include <Columns/ColumnTuple.h>
#include <Columns/ColumnMap.h>
#include <Columns/ColumnObject.h>
#include <Columns/ColumnsCommon.h>
#include <Columns/ColumnStringHelpers.h>
#include <Common/assert_cast.h>
#include <Common/quoteString.h>
#include <Core/AccurateComparison.h>
#include <Functions/IFunctionAdaptors.h>
#include <Functions/FunctionsMiscellaneous.h>
#include <Functions/FunctionHelpers.h>
#include <Functions/DateTimeTransforms.h>
#include <Functions/toFixedString.h>
#include <Functions/TransformDateTime64.h>
#include <Functions/FunctionsCodingIP.h>
#include <DataTypes/DataTypeLowCardinality.h>
#include <Columns/ColumnLowCardinality.h>