If there is no crush_choose_arg_map for a given pool, a NULL pointer is
passed to preserve existing crush_do_rule() behavior.

Reflects ceph.git commits 55fb91d64071552ea1bc65ab4ea84d3c8b73ab4b,
                          dbe36e08be00c6519a8c89718dd47b0219c20516.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>