**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 IFunction::Monotonicity get(const IDataType & type, const Field & left, const Field & right)
    {
        auto which = WhichDataType(type);
        if (which.isDateOrDate32() || which.isDateTime() || which.isDateTime64() || which.isInt8() || which.isInt16() || which.isUInt8() || which.isUInt16())
            return { .is_monotonic = true, .is_always_monotonic = true };
        else if (
            (which.isUInt() && ((left.isNull() || left.get<UInt64>() < 0xFFFF) && (right.isNull() || right.get<UInt64>() >= 0xFFFF)))
            || (which.isInt() && ((left.isNull() || left.get<Int64>() < 0xFFFF) && (right.isNull() || right.get<Int64>() >= 0xFFFF)))
            || (which.isFloat() && ((left.isNull() || left.get<Float64>() < 0xFFFF) && (right.isNull() || right.get<Float64>() >= 0xFFFF)))
            || !type.isValueRepresentedByNumber())
            return {};
        else
            return { .is_monotonic = true, .is_always_monotonic = true };
    }
};
struct ToDateTimeMonotonicity
{
    static bool has() { return true; }