**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 isDeterministic() const override { return true; }
    bool isDeterministicInScopeOfQuery() const override { return true; }
    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & ) const override { return true; }
    bool hasInformationAboutMonotonicity() const override
    {
        return static_cast<bool>(monotonicity_for_range);
    }
    Monotonicity getMonotonicityForRange(const IDataType & type, const Field & left, const Field & right) const override
    {
        return monotonicity_for_range(type, left, right);
    }
private:
    const char * cast_name;
    MonotonicityForRange monotonicity_for_range;
    DataTypes argument_types;
    DataTypePtr return_type;
    std::optional<Diagnostic> diagnostic;
    CastType cast_type;
    bool cast_ipv4_ipv6_default_on_conversion_error;
    static WrapperType createFunctionAdaptor(FunctionPtr function, const DataTypePtr & from_type)