pgsql: Allow PL/Python functions to return void, per gripe from James

Log Message:
-----------
Allow PL/Python functions to return void, per gripe from James Robinson
(I didn't use his patch, however). A void-returning PL/Python function
must return None (from Python), which is translated into a void datum
(and *not* NULL) for Postgres. I also added some regression tests for
this functionality.

Modified Files:
--------------
pgsql/src/pl/plpython:
plpython.c (r1.71 -> r1.72)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c.diff?r1=1.71&r2=1.72)
pgsql/src/pl/plpython/expected:
plpython_function.out (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_function.out.diff?r1=1.3&r2=1.4)
plpython_test.out (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_test.out.diff?r1=1.1&r2=1.2)
pgsql/src/pl/plpython/sql:
plpython_function.sql (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/sql/plpython_function.sql.diff?r1=1.3&r2=1.4)
plpython_test.sql (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/sql/plpython_test.sql.diff?r1=1.1&r2=1.2)