**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+
{
            const auto & from_names = from_type->getElementNames();
            std::unordered_map<String, size_t> from_positions;
            from_positions.reserve(from_names.size());
            for (size_t i = 0; i < from_names.size(); ++i)
                from_positions[from_names[i]] = i;
            const auto & to_names = to_type->getElementNames();
            element_wrappers.reserve(to_names.size());
            to_reverse_index.reserve(from_names.size());
            for (size_t i = 0; i < to_names.size(); ++i)
            {
                auto it = from_positions.find(to_names[i]);
                if (it != from_positions.end())
                {
                    element_wrappers.emplace_back(prepareUnpackDictionaries(from_element_types[it->second], to_element_types[i]));
                    to_reverse_index.emplace_back(it->second);
                }
                else
                {
                    element_wrappers.emplace_back();
                    to_reverse_index.emplace_back();
                }
            }
        }
        else
        {
            if (from_element_types.size() != to_element_types.size())
                throw Exception{"CAST AS Tuple can only be performed between tuple types with the same number of elements or from String.\n"
                    "Left type: " + from_type->getName() + ", right type: " + to_type->getName(), ErrorCodes::TYPE_MISMATCH};
            element_wrappers = getElementWrappers(from_element_types, to_element_types);