**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+
{
        bool to_nullable_string = to_nullable && std::is_same_v<ToDataType, DataTypeString>;
        return checked_return_type && !to_nullable_string;
    }
    bool useDefaultImplementationForConstants() const override { return true; }
    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override
    {
        if constexpr (std::is_same_v<ToDataType, DataTypeDateTime64>)
            return {2};
        return {1};
    }
    bool canBeExecutedOnDefaultArguments() const override { return false; }
    ColumnPtr executeImpl(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t input_rows_count) const override
    {
        try
        {
            return executeInternal(arguments, result_type, input_rows_count);
        }
        catch (Exception & e)
        {
            
            if (e.code() == ErrorCodes::ATTEMPT_TO_READ_AFTER_EOF)
            {
                e.addMessage("Cannot parse "
                    + result_type->getName() + " from "
                    + arguments[0].type->getName()