As you can see in an XXX comment in the libpq test program, the current implementation has the behavior that PQgetResult() returns NULL after a batch is finished and has reported PGRES_BATCH_END. I don't know if there's a hard reason to do that, but I'd like to supress it because it seems weird and out of place.
-- Álvaro Herrera 39°49'30"S 73°17'W