**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+
for (size_t i = 0; i < input_rows_count; ++i)
            {
                if constexpr (std::is_same_v<ToDataType, DataTypeUInt8>)
                {
                    if (result_is_bool)
                    {
                        vec_to[i] = vec_from[i] != FromFieldType(0);
                        continue;
                    }
                }
                if constexpr (std::is_same_v<FromDataType, DataTypeUUID> != std::is_same_v<ToDataType, DataTypeUUID>)
                {
                    throw Exception("Conversion between numeric types and UUID is not supported. Probably the passed UUID is unquoted", ErrorCodes::NOT_IMPLEMENTED);
                }
                else
                {
                    if constexpr (IsDataTypeDecimal<FromDataType> || IsDataTypeDecimal<ToDataType>)
                    {
                        if constexpr (std::is_same_v<Additions, AccurateOrNullConvertStrategyAdditions>)
                        {
                            ToFieldType result;
                            bool convert_result = false;
                            if constexpr (IsDataTypeDecimal<FromDataType> && IsDataTypeDecimal<ToDataType>)
                                convert_result = tryConvertDecimals<FromDataType, ToDataType>(vec_from[i], col_from->getScale(), col_to->getScale(), result);
                            else if constexpr (IsDataTypeDecimal<FromDataType> && IsDataTypeNumber<ToDataType>)
                                convert_result = tryConvertFromDecimal<FromDataType, ToDataType>(vec_from[i], col_from->getScale(), result);
                            else if constexpr (IsDataTypeNumber<FromDataType> && IsDataTypeDecimal<ToDataType>)
                                convert_result = tryConvertToDecimal<FromDataType, ToDataType>(vec_from[i], col_to->getScale(), result);
                            if (convert_result)
                                vec_to[i] = result;