pgsql: Make ARRAY(SELECT ...) return an empty array, rather than a NULL,

Log Message:
-----------
Make ARRAY(SELECT ...) return an empty array, rather than a NULL, when the
sub-select returns zero rows. Per complaint from Jens Schicke. Since this
is more in the nature of a definition change than a bug, not back-patched.

Modified Files:
--------------
pgsql/src/backend/executor:
nodeSubplan.c (r1.89 -> r1.90)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeSubplan.c?r1=1.89&r2=1.90)
pgsql/src/backend/optimizer/plan:
subselect.c (r1.123 -> r1.124)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c?r1=1.123&r2=1.124)
pgsql/src/include/nodes:
primnodes.h (r1.132 -> r1.133)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/primnodes.h?r1=1.132&r2=1.133)