Fix psql describe commands' handling of ACL columns for old servers.

Commit d1379ebf4 carelessly broke printACLColumn for pre-9.4 servers,
by using the cardinality() function which we introduced in 9.4.
We expect psql's describe-related commands to work back to 9.2, so
this is bad.  Use the longstanding array_length() function instead.

Per report from Christoph Berg.  Back-patch to v17.

Discussion: https://postgr.es/m/ZvLXYglRS6hMMhtr@msg.df7cb.de

src/bin/psql/describe.c

@@ -6678,7 +6678,7 @@ printACLColumn(PQExpBuffer buf, const char *colname)
 {
 	appendPQExpBuffer(buf,
 					  "CASE"
-					  " WHEN pg_catalog.cardinality(%s) = 0 THEN '%s'"
+					  " WHEN pg_catalog.array_length(%s, 1) = 0 THEN '%s'"
 					  " ELSE pg_catalog.array_to_string(%s, E'\\n')"
 					  " END AS \"%s\"",
 					  colname, gettext_noop("(none)"),