**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+
auto make_custom_serialization_wrapper = [&, cast_ipv4_ipv6_default_on_conversion_error_value](const auto & types) -> bool
        {
            using Types = std::decay_t<decltype(types)>;
            using ToDataType = typename Types::RightType;
            using FromDataType = typename Types::LeftType;
            if constexpr (WhichDataType(FromDataType::type_id).isStringOrFixedString())
            {
                if (to_type->getCustomSerialization() && to_type->getCustomName())
                {
                    if (to_type->getCustomName()->getName() == "IPv4")
                    {
                        ret = [cast_ipv4_ipv6_default_on_conversion_error_value](
                                  ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable * column_nullable, size_t)
                            -> ColumnPtr
                        {
                            if (!WhichDataType(result_type).isUInt32())
                                throw Exception(ErrorCodes::TYPE_MISMATCH, "Wrong result type {}. Expected UInt32", result_type->getName());
                            const auto * null_map = column_nullable ? &column_nullable->getNullMapData() : nullptr;
                            if (cast_ipv4_ipv6_default_on_conversion_error_value)
                                return convertToIPv4<IPStringToNumExceptionMode::Default>(arguments[0].column, null_map);
                            else
                                return convertToIPv4<IPStringToNumExceptionMode::Throw>(arguments[0].column, null_map);
                        };
                        return true;
                    }
                    if (to_type->getCustomName()->getName() == "IPv6")
                    {
                        ret = [cast_ipv4_ipv6_default_on_conversion_error_value](
                                  ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable * column_nullable, size_t)
                            -> ColumnPtr