pgsql: Allow PL/Python functions to return void, per gripe from James - Mailing list pgsql-committers

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)