json: Parse NULL types too

Null values are allowed in JSON, json-c can parse them, we should just
ignore them, without a warning. This patch does just that, so things
like the following will now go through error-free:

{"MESSAGE": "hello", "something": null}

Reported-by: Nicolas Szalay <nico@rottenbytes.info>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>