pg_stat_get_replication_slot() marked not strict, crashes

Hi,

I'm working to increase the test coverage of pgstat related stuff higher (for
the shared memory stats patch, of course).

"Accidentally" noticed that
SELECT * FROM pg_stat_get_replication_slot(NULL);
crashes. This is present in HEAD and 14.

I guess we'll have to add a code-level check in 14 to deal with this?

pg_stat_get_subscription_stats() also is wrongly marked. But at least in the
trivial cases just returns bogus results (for 0/InvalidOid). That's only in
HEAD, so easy to deal with.

The other functions returned by
SELECT oid::regprocedure FROM pg_proc WHERE proname LIKE 'pg%stat%' AND pronargs > 0 AND NOT proisstrict;
look ok.

I wonder if we ought to make PG_GETARG_DATUM(n) assert that !PG_ARGISNULL(n)?
That'd perhaps make it easier to catch some of these...

It'd be nice to have a test in sanity check to just call each non-strict
function with NULL inputs automatically. But the potential side-effects
probably makes that not a realistic option?

Greetings,

Andres Freund