**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 [wrapper_cast_type, type_index, scale, to_type, requested_result_is_nullable]
            (ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable *column_nullable, size_t input_rows_count)
        {
            ColumnPtr result_column;
            auto res = callOnIndexAndDataType<ToDataType>(type_index, [&](const auto & types) -> bool
            {
                using Types = std::decay_t<decltype(types)>;
                using LeftDataType = typename Types::LeftType;
                using RightDataType = typename Types::RightType;
                if constexpr (IsDataTypeDecimalOrNumber<LeftDataType> && IsDataTypeDecimalOrNumber<RightDataType> && !std::is_same_v<DataTypeDateTime64, RightDataType>)
                {
                    if (wrapper_cast_type == CastType::accurate)
                    {
                        AccurateConvertStrategyAdditions additions;
                        additions.scale = scale;
                        result_column = ConvertImpl<LeftDataType, RightDataType, FunctionName>::execute(
                            arguments, result_type, input_rows_count, additions);
                        return true;
                    }
                    else if (wrapper_cast_type == CastType::accurateOrNull)
                    {
                        AccurateOrNullConvertStrategyAdditions additions;
                        additions.scale = scale;
                        result_column = ConvertImpl<LeftDataType, RightDataType, FunctionName>::execute(
                            arguments, result_type, input_rows_count, additions);
                        return true;
                    }
                }
                else if constexpr (std::is_same_v<LeftDataType, DataTypeString>)
                {