Wrong results when grouping on a column with NULLs

[alamb]

Describe the bug
Given a column with null values, if you group by that column the null values are lost

To Reproduce
Input:

 c1 
----
 0
 3
 
 1
 3
(5 rows)

And run SELECT COUNT(*), c1 FROM test GROUP BY c1

Actual result

 "+-----------------+----+",
 "| COUNT(UInt8(1)) | c1 |",
 "+-----------------+----+",
 "| 2 | 3 |",
 "| 2 | 0 |", <-- NOTE there is only a single actual value of 0 in the input (the other is NULL)
 "| 1 | 1 |",
 "+-----------------+----+",

Expected behavior
Here is the correct answer according to postgres (note the row for c1 is NULL):

alamb=# SELECT COUNT(*), c1 FROM test GROUP BY c1;
 count | c1 
-------+----
 1 | 
 2 | 3
 1 | 0
 1 | 1
(4 rows)

Additional context
Discovered while playing around with #781

Here is the entire reproducer in postgres

alamb=# drop table test;
DROP TABLE
alamb=# create table test (c1 int);
CREATE TABLE
alamb=# insert into test values(0);
INSERT 0 1
alamb=# insert into test values(3);
INSERT 0 1
alamb=# insert into test values(NULL);
INSERT 0 1
alamb=# insert into test values(1);
INSERT 0 1
alamb=# insert into test values(3);
INSERT 0 1
alamb=# SELECT COUNT(*), c1 FROM test GROUP BY c1;
 count | c1 
-------+----
 1 | 
 2 | 3
 1 | 0
 1 | 1
(4 rows)

[alamb]

Actually, it is more insidious -- note that

 "+-----------------+----+",
 "| COUNT(UInt8(1)) | c1 |",
 "+-----------------+----+",
 "| 2 | 3 |",
 "| 2 | 0 |", <-- NOTE there is only a single actual value of 0 in the input (the other is NULL)
 "| 1 | 1 |",
 "+-----------------+----+",```

[Dandandan]

Great find. I think group by doesn't support nulls at all ATM.

The groupby scalars don't have a null and extracting a key doesn't look yet at whether the value is null or not.

[alamb]

@Dandandan I think you are right. I will write up a proposal of how to fix this shortly.

[alamb]

Resolved in #793 (though we are working on improved performance in #790)