**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+
, MonotonicityForRange && monotonicity_for_range_
            , const DataTypes & argument_types_
            , const DataTypePtr & return_type_
            , std::optional<Diagnostic> diagnostic_
            , CastType cast_type_
            , bool cast_ipv4_ipv6_default_on_conversion_error_)
        : cast_name(cast_name_), monotonicity_for_range(std::move(monotonicity_for_range_))
        , argument_types(argument_types_), return_type(return_type_), diagnostic(std::move(diagnostic_))
        , cast_type(cast_type_)
        , cast_ipv4_ipv6_default_on_conversion_error(cast_ipv4_ipv6_default_on_conversion_error_)
    {
    }
    const DataTypes & getArgumentTypes() const override { return argument_types; }
    const DataTypePtr & getResultType() const override { return return_type; }
    ExecutableFunctionPtr prepare(const ColumnsWithTypeAndName & ) const override
    {
        try
        {
            return std::make_unique<ExecutableFunctionCast>(
                    prepareUnpackDictionaries(getArgumentTypes()[0], getResultType()), cast_name, diagnostic);
        }
        catch (Exception & e)
        {
            if (diagnostic)
                e.addMessage("while converting source column " + backQuoteIfNeed(diagnostic->column_from) +
                             " to destination column " + backQuoteIfNeed(diagnostic->column_to));
            throw;
        }
    }
    String getName() const override { return cast_name; }