**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+
for (size_t row = 0; row < offsets.size(); ++row)
            {
                for (size_t i = offsets[static_cast<ssize_t>(row) - 1]; i < offsets[row]; ++i)
                {
                    auto ref = key_column_str.getDataAt(i);
                    bool inserted;
                    SubcolumnsMap::LookupResult it;
                    subcolumns.emplace(ref, it, inserted);
                    auto & subcolumn = it->getMapped();
                    if (inserted)
                        subcolumn = value_column.cloneEmpty()->cloneResized(row);
                    
                    if (subcolumn->size() == row)
                        subcolumn->insertFrom(value_column, i);
                }
                
                for (const auto & [_, subcolumn] : subcolumns)
                    if (subcolumn->size() == row)
                        subcolumn->insertDefault();
            }
            auto column_object = ColumnObject::create(has_nullable_subcolumns);
            for (auto && [key, subcolumn] : subcolumns)
            {
                PathInData path(key.toView());
                column_object->addSubcolumn(path, std::move(subcolumn));
            }
            return column_object;
        };
    }
    WrapperType createObjectWrapper(const DataTypePtr & from_type, const DataTypeObject * to_type) const