**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+
convertFromTime<ToDataType>(vec_to[i],res);
                    }
                }
                else if constexpr (parsing_mode == ConvertFromStringParsingMode::BestEffortUS)
                {
                    if constexpr (to_datetime64)
                    {
                        DateTime64 res = 0;
                        parsed = tryParseDateTime64BestEffortUS(res, col_to->getScale(), read_buffer, *local_time_zone, *utc_time_zone);
                        vec_to[i] = res;
                    }
                    else
                    {
                        time_t res;
                        parsed = tryParseDateTimeBestEffortUS(res, read_buffer, *local_time_zone, *utc_time_zone);
                        convertFromTime<ToDataType>(vec_to[i],res);
                    }
                }
                else
                {
                    if constexpr (to_datetime64)
                    {
                        DateTime64 value = 0;
                        parsed = tryReadDateTime64Text(value, col_to->getScale(), read_buffer, *local_time_zone);
                        vec_to[i] = value;
                    }
                    else if constexpr (IsDataTypeDecimal<ToDataType>)
                    {
                        parsed = SerializationDecimal<typename ToDataType::FieldType>::tryReadText(
                            vec_to[i], read_buffer, ToDataType::maxPrecision(), col_to->getScale());