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

doc/src/sgml/ddl.sgml

@@ -1737,6 +1737,11 @@ ALTER TABLE products RENAME TO items;
    <primary>ACL</primary>
   </indexterm>
 
+  <indexterm zone="ddl-priv-default">
+   <primary>privilege</primary>
+   <secondary>default</secondary>
+  </indexterm>
+
   <para>
    When an object is created, it is assigned an owner. The
    owner is normally the role that executed the creation statement.
@@ -2049,7 +2054,7 @@ REVOKE ALL ON accounts FROM PUBLIC;
    reference page of the respective command.
   </para>
 
-  <para>
+  <para id="ddl-priv-default">
    PostgreSQL grants privileges on some types of objects to
    <literal>PUBLIC</literal> by default when the objects are created.
    No privileges are granted to <literal>PUBLIC</literal> by default on
@@ -2375,6 +2380,15 @@ GRANT SELECT (col1), UPDATE (col1) ON mytable TO miriam_rw;
    access privileges display.  A <literal>*</literal> will appear only when
    grant options have been explicitly granted to someone.
   </para>
+
+  <para>
+   The <quote>Access privileges</quote> column
+   shows <literal>(none)</literal> when the object's privileges entry is
+   non-null but empty.  This means that no privileges are granted at all,
+   even to the object's owner &mdash; a rare situation.  (The owner still
+   has implicit grant options in this case, and so could re-grant her own
+   privileges; but she has none at the moment.)
+  </para>
  </sect1>
 
  <sect1 id="ddl-rowsecurity">