**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+
else
                            {
                                vec_to[i] = static_cast<ToFieldType>(0);
                                (*vec_null_map_to)[i] = true;
                            }
                        }
                        else
                        {
                            if constexpr (IsDataTypeDecimal<FromDataType> && IsDataTypeDecimal<ToDataType>)
                                vec_to[i] = convertDecimals<FromDataType, ToDataType>(vec_from[i], col_from->getScale(), col_to->getScale());
                            else if constexpr (IsDataTypeDecimal<FromDataType> && IsDataTypeNumber<ToDataType>)
                                vec_to[i] = convertFromDecimal<FromDataType, ToDataType>(vec_from[i], col_from->getScale());
                            else if constexpr (IsDataTypeNumber<FromDataType> && IsDataTypeDecimal<ToDataType>)
                                vec_to[i] = convertToDecimal<FromDataType, ToDataType>(vec_from[i], col_to->getScale());
                            else
                                throw Exception("Unsupported data type in conversion function", ErrorCodes::CANNOT_CONVERT_TYPE);
                        }
                    }
                    else
                    {
                        
                        if constexpr (std::is_floating_point_v<FromFieldType> && !std::is_floating_point_v<ToFieldType>)
                        {
                            if (!isFinite(vec_from[i]))
                            {
                                if constexpr (std::is_same_v<Additions, AccurateOrNullConvertStrategyAdditions>)
                                {
                                    vec_to[i] = 0;
                                    (*vec_null_map_to)[i] = true;
                                    continue;