plpython: fix for improperly handled NULL arguments in prepared plans

The attached patch fixes a bug reported by Kevin Jacobs

> When we create a function like so:
>
> CREATE OR REPLACE FUNCTION jojo (timestamp) RETURNS varchar AS '
> pln = plpy.prepare("UPDATE jtab SET jfld = $1",["timestamp"])
> rcs = plpy.execute(pln,[args[0]])
> return "YES"' LANGUAGE 'PLPYTHON';
>
> SELECT jojo(now());
> works fine but
>
> SELECT jojo(NULL);
> errors out with
> NOTICE: plpython: in function __plpython_procedure_jojo_49818349:
> plpy.Error: Unknown error in PLy_spi_execute_plan
> ERROR: Bad timestamp external representation 'None'

NULL arguments were improperly converted to the string 'None' rather than being marked
as NULLs in the third argument to SPI_execp. The patch is against CVS HEAD.

--
Andrew Bosma <bosma(at)epigenomix(dot)com>