**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+
if (const ColVecFrom * col_from = checkAndGetColumn<ColVecFrom>(named_from.column.get()))
        {
            typename ColVecTo::MutablePtr col_to = nullptr;
            if constexpr (IsDataTypeDecimal<ToDataType>)
            {
                UInt32 scale;
                if constexpr (std::is_same_v<Additions, AccurateConvertStrategyAdditions>
                    || std::is_same_v<Additions, AccurateOrNullConvertStrategyAdditions>)
                {
                    scale = additions.scale;
                }
                else
                {
                    scale = additions;
                }
                col_to = ColVecTo::create(0, scale);
            }
            else
                col_to = ColVecTo::create();
            const auto & vec_from = col_from->getData();
            auto & vec_to = col_to->getData();
            vec_to.resize(input_rows_count);
            ColumnUInt8::MutablePtr col_null_map_to;
            ColumnUInt8::Container * vec_null_map_to [[maybe_unused]] = nullptr;
            if constexpr (std::is_same_v<Additions, AccurateOrNullConvertStrategyAdditions>)
            {
                col_null_map_to = ColumnUInt8::create(input_rows_count, false);
                vec_null_map_to = &col_null_map_to->getData();
            }
            bool result_is_bool = isBool(result_type);