**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+
static FunctionPtr create() { return std::make_shared<FunctionConvert>(); }
    String getName() const override
    {
        return name;
    }
    bool isVariadic() const override { return true; }
    size_t getNumberOfArguments() const override { return 0; }
    bool isInjective(const ColumnsWithTypeAndName &) const override { return std::is_same_v<Name, NameToString>; }
    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & arguments) const override
    {
        
        return !(to_date_or_datetime && isNumber(*arguments[0].type));
    }
    using DefaultReturnTypeGetter = std::function<DataTypePtr(const ColumnsWithTypeAndName &)>;
    static DataTypePtr getReturnTypeDefaultImplementationForNulls(const ColumnsWithTypeAndName & arguments, const DefaultReturnTypeGetter & getter)
    {
        NullPresence null_presence = getNullPresense(arguments);
        if (null_presence.has_null_constant)
        {
            return makeNullable(std::make_shared<DataTypeNothing>());
        }
        if (null_presence.has_nullable)
        {
            auto nested_columns = Block(createBlockWithNestedColumns(arguments));
            auto return_type = getter(ColumnsWithTypeAndName(nested_columns.begin(), nested_columns.end()));
            return makeNullable(return_type);
        }
        return getter(arguments);
    }
    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override