**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 (!isStringOrFixedString(from_type))
            throw Exception{"CAST AS FixedString is only implemented for types String and FixedString", ErrorCodes::NOT_IMPLEMENTED};
        bool exception_mode_null = cast_type == CastType::accurateOrNull;
        return [exception_mode_null, N] (ColumnsWithTypeAndName & arguments, const DataTypePtr &, const ColumnNullable *, size_t )
        {
            if (exception_mode_null)
                return FunctionToFixedString::executeForN<ConvertToFixedStringExceptionMode::Null>(arguments, N);
            else
                return FunctionToFixedString::executeForN<ConvertToFixedStringExceptionMode::Throw>(arguments, N);
        };
    }
    template <typename ToDataType>
    requires IsDataTypeDecimal<ToDataType>
    WrapperType createDecimalWrapper(const DataTypePtr & from_type, const ToDataType * to_type, bool requested_result_is_nullable) const
    {
        TypeIndex type_index = from_type->getTypeId();
        UInt32 scale = to_type->getScale();
        WhichDataType which(type_index);
        bool ok = which.isNativeInt() || which.isNativeUInt() || which.isDecimal() || which.isFloat() || which.isDateOrDate32() || which.isDateTime() || which.isDateTime64()
            || which.isStringOrFixedString();
        if (!ok)
        {
            if (cast_type == CastType::accurateOrNull)
                return createToNullableColumnWrapper();
            else
                throw Exception{"Conversion from " + from_type->getName() + " to " + to_type->getName() + " is not supported",
                    ErrorCodes::CANNOT_CONVERT_TYPE};
        }
        auto wrapper_cast_type = cast_type;