**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+
: DateTimeTransformImpl<DataTypeFloat32, DataTypeDateTime64, ToDateTime64TransformFloat<DataTypeFloat32, Float32>> {};
template <typename Name> struct ConvertImpl<DataTypeFloat64, DataTypeDateTime64, Name>
    : DateTimeTransformImpl<DataTypeFloat64, DataTypeDateTime64, ToDateTime64TransformFloat<DataTypeFloat64, Float64>> {};
/** Conversion of DateTime64 to Date or DateTime: discards fractional part.
 */
template <typename Transform>
struct FromDateTime64Transform
{
    static constexpr auto name = Transform::name;
    const DateTime64::NativeType scale_multiplier = 1;
    FromDateTime64Transform(UInt32 scale) 
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}
    inline auto execute(DateTime64::NativeType dt, const DateLUTImpl & time_zone) const
    {
        const auto c = DecimalUtils::splitWithScaleMultiplier(DateTime64(dt), scale_multiplier);
        return Transform::execute(static_cast<UInt32>(c.whole), time_zone);
    }
};
template <typename Name> struct ConvertImpl<DataTypeDateTime64, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDateTime64, DataTypeDate, TransformDateTime64<ToDateImpl>> {};
template <typename Name> struct ConvertImpl<DataTypeDateTime64, DataTypeDateTime, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDateTime64, DataTypeDateTime, TransformDateTime64<ToDateTimeImpl>> {};
struct ToDateTime64Transform
{
    static constexpr auto name = "toDateTime64";
    const DateTime64::NativeType scale_multiplier = 1;
    ToDateTime64Transform(UInt32 scale = 0) 
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}