**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+
{
        return from;
    }
};
/** Special case of converting Int8, Int16, (U)Int32 or (U)Int64 (and also, for convenience,
  * Float32, Float64) to Date. If the number is negative, saturate it to unix epoch time. If the
  * number is less than 65536, then it is treated as DayNum, and if it's greater or equals to 65536,
  * then treated as unix timestamp. If the number exceeds UInt32, saturate to MAX_UINT32 then as DayNum.
  * It's a bit illogical, as we actually have two functions in one.
  * But allows to support frequent case,
  *  when user write toDate(UInt32), expecting conversion of unix timestamp to Date.
  *  (otherwise such usage would be frequent mistake).
  */
template <typename Name> struct ConvertImpl<DataTypeUInt32, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeUInt32, DataTypeDate, ToDateTransform32Or64<UInt32, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeUInt64, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeUInt64, DataTypeDate, ToDateTransform32Or64<UInt64, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeInt8, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeInt8, DataTypeDate, ToDateTransform8Or16Signed<Int8, UInt16>> {};
template <typename Name> struct ConvertImpl<DataTypeInt16, DataTypeDate, Name, ConvertDefaultBehaviorTag>