**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+
WhichDataType which(from_type_index);
        bool can_apply_accurate_cast = (cast_type == CastType::accurate || cast_type == CastType::accurateOrNull)
            && (which.isInt() || which.isUInt() || which.isFloat());
        if (requested_result_is_nullable && checkAndGetDataType<DataTypeString>(from_type.get()))
        {
            
            
            FunctionPtr function = FunctionConvertFromString<ToDataType, FunctionName, ConvertFromStringExceptionMode::Null>::create();
            return createFunctionAdaptor(function, from_type);
        }
        else if (!can_apply_accurate_cast)
        {
            FunctionPtr function = FunctionTo<ToDataType>::Type::create();
            return createFunctionAdaptor(function, from_type);
        }
        auto wrapper_cast_type = cast_type;
        return [wrapper_cast_type, from_type_index, to_type]
            (ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, const ColumnNullable *column_nullable, size_t input_rows_count)
        {
            ColumnPtr result_column;
            auto res = callOnIndexAndDataType<ToDataType>(from_type_index, [&](const auto & types) -> bool {
                using Types = std::decay_t<decltype(types)>;
                using LeftDataType = typename Types::LeftType;
                using RightDataType = typename Types::RightType;
                if constexpr (IsDataTypeNumber<LeftDataType> && IsDataTypeNumber<RightDataType>)
                {
                    if (wrapper_cast_type == CastType::accurate)
                    {
                        result_column = ConvertImpl<LeftDataType, RightDataType, FunctionName>::execute(
                            arguments, result_type, input_rows_count, AccurateConvertStrategyAdditions());