Unexpected behavior of groupConcat function when using null in expression

https://fiddle.clickhouse.com/6d549ea8-70c0-4d27-a80d-6ad0bee88f9c

```sql
create table t (a Nullable(String)) engine MergeTree order by ();

insert into t values ('123'), ('456'), ('789');

select groupConcat(',', 1, 1, 1, 1)(a, null, a, a, 1, 1) from t;
```

Output:
```
\N
```

Why I do not get an exception executing this invalid query?

I expect to see either

```
DB::Exception: Incorrect number of parameters for aggregate function groupConcat, should be 0, 1 or 2, got: 5. (TOO_MANY_ARGUMENTS_FOR_FUNCTION)
```
or
```
DB::Exception: Aggregate function groupConcat requires single argument. (NUMBER_OF_ARGUMENTS_DOESNT_MATCH)
```