BUG #18997: Two equivalent queries return different results

The following bug has been logged on the website:

Bug reference:      18997
Logged by:          Jinhui Lai
Email address:      jinhui-lai@foxmail.com
PostgreSQL version: 17.5
Operating system:   ubuntu 22.04
Description:        

Dear PG developers:

Thanks for reading my report.

I think these "SELECT COUNT(c0) FILTER (WHERE c0 IS NOT NULL) FROM t0" and
"SELECT COUNT(c0) FROM t0 WHERE c0 IS NOT NULL;" are equivalent. However,
their results are different.

Please you can reproduce it as follows:

CREATE TYPE composite AS (a float8, b float8);
CREATE TABLE t0 (c0 composite NOT NULL);
INSERT INTO t0 VALUES ('(,)');

test=# SELECT COUNT(c0) FILTER (WHERE c0 IS NOT NULL) FROM t0;
 count
-------
     0
(1 row)

test=# SELECT COUNT(c0) FROM t0 WHERE c0 IS NOT NULL;
 count
-------
     1
(1 row)

Moreover, when I execute this case in the former versions, such as pg 16,
15... these  two queries return the same result.

Best regard,
Jinhui