**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+
inline DateTime64::NativeType execute(UInt16 d, const DateLUTImpl & time_zone) const
    {
        const auto dt = ToDateTimeImpl::execute(d, time_zone);
        return execute(dt, time_zone);
    }
    inline DateTime64::NativeType execute(Int32 d, const DateLUTImpl & time_zone) const
    {
        const auto dt = time_zone.fromDayNum(ExtendedDayNum(d));
        return DecimalUtils::decimalFromComponentsWithMultiplier<DateTime64>(dt, 0, scale_multiplier);
    }
    inline DateTime64::NativeType execute(UInt32 dt, const DateLUTImpl & ) const
    {
        return DecimalUtils::decimalFromComponentsWithMultiplier<DateTime64>(dt, 0, scale_multiplier);
    }
};
/** Conversion of Date or DateTime to DateTime64: add zero sub-second part.
  */
template <typename Name> struct ConvertImpl<DataTypeDate, DataTypeDateTime64, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDate, DataTypeDateTime64, ToDateTime64Transform> {};
template <typename Name> struct ConvertImpl<DataTypeDate32, DataTypeDateTime64, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDate32, DataTypeDateTime64, ToDateTime64Transform> {};
template <typename Name> struct ConvertImpl<DataTypeDateTime, DataTypeDateTime64, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDateTime, DataTypeDateTime64, ToDateTime64Transform> {};
/** Transformation of numbers, dates, datetimes to strings: through formatting.
  */
template <typename DataType>
struct FormatImpl
{
    template <typename ReturnType = void>
    static ReturnType execute(const typename DataType::FieldType x, WriteBuffer & wb, const DataType *, const DateLUTImpl *)