pgsql: When we added the ability to have zero-element ARRAY[] constructs

Log Message:
-----------
When we added the ability to have zero-element ARRAY[] constructs by adding an
explicit cast to show the intended array type, we forgot to teach ruleutils.c
to print out such constructs properly. Found by noting bogus output from
recent changes in polymorphism regression test.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
ruleutils.c (r1.289 -> r1.290)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c?r1=1.289&r2=1.290)
pgsql/src/test/regress/expected:
polymorphism.out (r1.17 -> r1.18)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/polymorphism.out?r1=1.17&r2=1.18)