**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)
    {
        IFunction::Monotonicity positive{ .is_monotonic = true };
        IFunction::Monotonicity not_monotonic;
        const auto * type_ptr = &type;
        if (const auto * low_cardinality_type = checkAndGetDataType<DataTypeLowCardinality>(type_ptr))
            type_ptr = low_cardinality_type->getDictionaryType().get();
        
        if (checkDataTypes<DataTypeDate, DataTypeDate32, DataTypeDateTime, DataTypeString>(type_ptr))
            return positive;
        if (left.isNull() || right.isNull())
            return {};
        if (left.getType() == Field::Types::UInt64
            && right.getType() == Field::Types::UInt64)
        {
            return (left.get<Int64>() == 0 && right.get<Int64>() == 0)
                || (floor(log10(left.get<UInt64>())) == floor(log10(right.get<UInt64>())))
                ? positive : not_monotonic;
        }
        if (left.getType() == Field::Types::Int64
            && right.getType() == Field::Types::Int64)
        {
            return (left.get<Int64>() == 0 && right.get<Int64>() == 0)
                || (left.get<Int64>() > 0 && right.get<Int64>() > 0 && floor(log10(left.get<Int64>())) == floor(log10(right.get<Int64>())))
                ? positive : not_monotonic;
        }
        return not_monotonic;
    }