Should the function get_variable_numdistinct consider the case when stanullfrac is 1.0?

Hi hackers,

It seems the function `get_variable_numdistinct` ignore the case when stanullfrac is 1.0:

# create table t(a int, b int);
CREATE TABLE
# insert into t select i from generate_series(1, 10000)i;
INSERT 0 10000
gpadmin=# analyze t;
ANALYZE
# explain analyze select b, count(1) from t group by b;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
HashAggregate (cost=195.00..197.00 rows=200 width=12) (actual time=5.928..5.930 rows=1 loops=1)
Group Key: b
Batches: 1 Memory Usage: 40kB
-> Seq Scan on t (cost=0.00..145.00 rows=10000 width=4) (actual time=0.018..1.747 rows=10000 loops=1)
Planning Time: 0.237 ms
Execution Time: 5.983 ms
(6 rows)

So it gives the estimate using the default value: 200.

I have added some lines of code to take `stanullfrac ==1.0` into account. With the patch attached, we now get:

# explain analyze select b, count(1) from t group by b;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
HashAggregate (cost=195.00..195.01 rows=1 width=12) (actual time=6.163..6.164 rows=1 loops=1)
Group Key: b
Batches: 1 Memory Usage: 24kB
-> Seq Scan on t (cost=0.00..145.00 rows=10000 width=4) (actual time=0.024..1.823 rows=10000 loops=1)
Planning Time: 0.535 ms
Execution Time: 6.344 ms
(6 rows)

I am not sure if this change is valuable in practical env, but it should go in the correct direction.

Any comments on this are appreciated.