**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+
*  (Date is represented internally as number of days from some day; DateTime - as unix timestamp)
  */
template <typename FromDataType, typename ToDataType, typename Name, typename SpecialTag = ConvertDefaultBehaviorTag>
struct ConvertImpl
{
    using FromFieldType = typename FromDataType::FieldType;
    using ToFieldType = typename ToDataType::FieldType;
    template <typename Additions = void *>
    static ColumnPtr NO_SANITIZE_UNDEFINED execute(
        const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type [[maybe_unused]], size_t input_rows_count,
        Additions additions [[maybe_unused]] = Additions())
    {
        const ColumnWithTypeAndName & named_from = arguments[0];
        using ColVecFrom = typename FromDataType::ColumnType;
        using ColVecTo = typename ToDataType::ColumnType;
        if (std::is_same_v<Name, NameToUnixTimestamp>)
        {
            if (isDateOrDate32(named_from.type))
                throw Exception("Illegal type " + named_from.type->getName() + " of first argument of function " + Name::name,
                    ErrorCodes::ILLEGAL_COLUMN);
        }
        if constexpr ((IsDataTypeDecimal<FromDataType> || IsDataTypeDecimal<ToDataType>)
            && !(std::is_same_v<DataTypeDateTime64, FromDataType> || std::is_same_v<DataTypeDateTime64, ToDataType>))
        {
            if constexpr (!IsDataTypeDecimalOrNumber<FromDataType> || !IsDataTypeDecimalOrNumber<ToDataType>)
            {
                throw Exception("Illegal column " + named_from.column->getName() + " of first argument of function " + Name::name,
                    ErrorCodes::ILLEGAL_COLUMN);
            }
        }