**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+
struct NameToDateTime32 { static constexpr auto name = "toDateTime32"; };
struct NameToDateTime64 { static constexpr auto name = "toDateTime64"; };
struct NameToString { static constexpr auto name = "toString"; };
struct NameToDecimal32 { static constexpr auto name = "toDecimal32"; };
struct NameToDecimal64 { static constexpr auto name = "toDecimal64"; };
struct NameToDecimal128 { static constexpr auto name = "toDecimal128"; };
struct NameToDecimal256 { static constexpr auto name = "toDecimal256"; };
#define DEFINE_NAME_TO_INTERVAL(INTERVAL_KIND) \
    struct NameToInterval ## INTERVAL_KIND \
    { \
        static constexpr auto name = "toInterval" #INTERVAL_KIND; \
        static constexpr auto kind = IntervalKind::INTERVAL_KIND; \
    };
DEFINE_NAME_TO_INTERVAL(Nanosecond)
DEFINE_NAME_TO_INTERVAL(Microsecond)
DEFINE_NAME_TO_INTERVAL(Millisecond)
DEFINE_NAME_TO_INTERVAL(Second)
DEFINE_NAME_TO_INTERVAL(Minute)
DEFINE_NAME_TO_INTERVAL(Hour)
DEFINE_NAME_TO_INTERVAL(Day)
DEFINE_NAME_TO_INTERVAL(Week)
DEFINE_NAME_TO_INTERVAL(Month)
DEFINE_NAME_TO_INTERVAL(Quarter)
DEFINE_NAME_TO_INTERVAL(Year)
#undef DEFINE_NAME_TO_INTERVAL
struct NameParseDateTimeBestEffort;
struct NameParseDateTimeBestEffortOrZero;
struct NameParseDateTimeBestEffortOrNull;
template<typename Name, typename ToDataType>
static inline bool isDateTime64(const ColumnsWithTypeAndName & arguments)