actions: Make "arp { drop; };" acceptable.

actions: Make "arp { drop; };" acceptable.

Before this commit, the OVN action parser would accept "arp {};" and then
the formatter would format it back as "arp { drop; };", but the parser
didn't accept the latter.  There were basically two choices: make the
parser accept "arp { drop; };" or make the formatter output "arp {};"
(or both).  This patch does (only) the former, and adds a test to avoid
regression.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Mickey Spiegel <mickeys.dev@gmail.com>