Fix output of zero privileges in psql

I wrote a patch to change psql's display of zero privileges after a user's
reported confusion with the psql output for zero vs. default privileges [1].
Admittedly, zero privileges is a rare use case [2] but I think psql should not
confuse the user in the off chance that this happens.

With this change psql now prints "(none)" for zero privileges instead of
nothing. This affects the following meta commands:

\db+ \dD+ \df+ \dL+ \dl+ \dn+ \dp \dT+ \l

Default privileges start as NULL::aclitem[] in various catalog columns but
revoking the default privileges leaves an empty aclitem array. Using
\pset null '(null)' as a workaround to spot default privileges does not work
because the meta commands ignore this setting.

The privileges shown by \dconfig+ and \ddp as well as the column privileges
shown by \dp are not affected by this change because those privileges are reset
to NULL instead of leaving empty arrays.

Commands \des+ and \dew+ are not covered in src/test/regress because no foreign
data wrapper is available at this point to create a foreign server.

[1] https://www.postgresql.org/message-id/efdd465d-a795-6188-7f71-7cdb4b2be031%40mtneva.com
[2] https://www.postgresql.org/message-id/31246.1693337238%40sss.pgh.pa.us

--
Erik