BUG #19615: COVAR_POP / COVAR_SAMP / REGR_SXY return 0.0 instead of NaN

The following bug has been logged on the website:

Bug reference: 19615
Logged by: Junwen An
Email address: feasiblechart(at)gmail(dot)com
PostgreSQL version: 19beta2
Operating system: Linux Ubuntu
Description:

I found `COVAR_POP`/`REGR_SXY` returns 0.0 instead of NaN when one arg is
constant and the other has Inf (not first), which might be unexpected. I
could reproduce it on 19beta1, 19beta2, and 20devel, but not on 18.4.

Minimal repro:

CREATE TABLE t (y double precision);
INSERT INTO t VALUES (3), ('Infinity'), (4);

SELECT COVAR_POP(0::float8, y) FROM t;
-- Expected 1 row: NaN
-- Actual: 0.0

SELECT COVAR_POP(y, 0::float8) FROM t;
-- Expected 1 row: NaN
-- Actual: 0.0

SELECT COVAR_SAMP(0::float8, y) FROM t;
-- Expected 1 row: NaN
-- Actual: 0.0

SELECT REGR_SXY(0::float8, y) FROM t;
-- Expected 1 row: NaN
-- Actual: 0.0

Did some further experiments, and it seems this behavior also depends on the
position of 'Inf'

WITH t(ord, y) AS (
VALUES
(1, 'Infinity'::float8),
(2, 3::float8),
(3, 4::float8)
)
SELECT
covar_pop(0::float8, y ORDER BY ord) AS inf_first,
covar_pop(0::float8, y ORDER BY ord DESC) AS inf_last
FROM t;

inf_first | inf_last
-----------+----------
NaN | 0