Improve default and empty privilege outputs in psql.

Default privileges are represented as NULL::aclitem[] in catalog ACL
columns, while revoking all privileges leaves an empty aclitem[].
These two cases used to produce identical output in psql meta-commands
like \dp.  Using something like "\pset null '(default)'" as a
workaround for spotting the difference did not work, because null
values were always displayed as empty strings by describe.c's
meta-commands.

This patch improves that with two changes:

1. Print "(none)" for empty privileges so that the user is able to
   distinguish them from default privileges, even without special
   workarounds.

2. Remove the special handling of null values in describe.c,
   so that "\pset null" is honored like everywhere else.
   (This affects all output from these commands, not only ACLs.)

The privileges shown by \dconfig+ and \ddp as well as the column
privileges shown by \dp are not affected by change #1, because the
respective aclitem[] is reset to NULL or deleted from the catalog
instead of leaving an empty array.

Erik Wienhold and Laurenz Albe

Discussion: https://postgr.es/m/1966228777.127452.1694979110595@office.mailbox.org

src/test/regress/sql/psql.sql

@@ -1578,6 +1578,7 @@ COMMIT;
 SELECT COUNT(*) AS "#mum"
 FROM bla WHERE s = 'Mum' \;               -- no mum here
 SELECT * FROM bla ORDER BY 1;
+COMMIT;
 
 -- reset all
 \set AUTOCOMMIT on
@@ -1853,3 +1854,34 @@ DROP ROLE regress_du_role0;
 DROP ROLE regress_du_role1;
 DROP ROLE regress_du_role2;
 DROP ROLE regress_du_admin;
+
+-- Test display of empty privileges.
+BEGIN;
+-- Create an owner for tested objects because output contains owner name.
+CREATE ROLE regress_zeropriv_owner;
+SET LOCAL ROLE regress_zeropriv_owner;
+
+CREATE DOMAIN regress_zeropriv_domain AS int;
+REVOKE ALL ON DOMAIN regress_zeropriv_domain FROM CURRENT_USER, PUBLIC;
+\dD+ regress_zeropriv_domain
+
+CREATE PROCEDURE regress_zeropriv_proc() LANGUAGE sql AS '';
+REVOKE ALL ON PROCEDURE regress_zeropriv_proc() FROM CURRENT_USER, PUBLIC;
+\df+ regress_zeropriv_proc
+
+CREATE TABLE regress_zeropriv_tbl (a int);
+REVOKE ALL ON TABLE regress_zeropriv_tbl FROM CURRENT_USER;
+\dp regress_zeropriv_tbl
+
+CREATE TYPE regress_zeropriv_type AS (a int);
+REVOKE ALL ON TYPE regress_zeropriv_type FROM CURRENT_USER, PUBLIC;
+\dT+ regress_zeropriv_type
+
+ROLLBACK;
+
+-- Test display of default privileges with \pset null.
+CREATE TABLE defprivs (a int);
+\pset null '(default)'
+\z defprivs
+\pset null ''
+DROP TABLE defprivs;