**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+
{
        if (from < 0)
            return 0;
        return std::min<Int64>(Int64(from), Int64(0xFFFFFFFF));
    }
};
/** Special case of converting Int8, Int16, Int32 or (U)Int64 (and also, for convenience, Float32,
  * Float64) to DateTime. If the number is negative, saturate it to unix epoch time. If the number
  * exceeds UInt32, saturate to MAX_UINT32.
  */
template <typename Name> struct ConvertImpl<DataTypeInt8, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeInt8, DataTypeDateTime, ToDateTimeTransformSigned<Int8, UInt32>> {};
template <typename Name> struct ConvertImpl<DataTypeInt16, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeInt16, DataTypeDateTime, ToDateTimeTransformSigned<Int16, UInt32>> {};
template <typename Name> struct ConvertImpl<DataTypeInt32, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeInt32, DataTypeDateTime, ToDateTimeTransformSigned<Int32, UInt32>> {};
template <typename Name> struct ConvertImpl<DataTypeInt64, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeInt64, DataTypeDateTime, ToDateTimeTransform64Signed<Int64, UInt32>> {};
template <typename Name> struct ConvertImpl<DataTypeUInt64, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeUInt64, DataTypeDateTime, ToDateTimeTransform64<UInt64, UInt32>> {};
template <typename Name> struct ConvertImpl<DataTypeFloat32, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeFloat32, DataTypeDateTime, ToDateTimeTransform64Signed<Float32, UInt32>> {};
template <typename Name> struct ConvertImpl<DataTypeFloat64, DataTypeDateTime, Name>
    : DateTimeTransformImpl<DataTypeFloat64, DataTypeDateTime, ToDateTimeTransform64Signed<Float64, UInt32>> {};
const time_t LUT_MIN_TIME = -2208988800l;           
const time_t LUT_MAX_TIME = 10413791999l;           
/** Conversion of numeric to DateTime64
  */
template <typename FromType>
struct ToDateTime64TransformUnsigned