activity/post.json API should reject empty post (fixes #2698)
activity/post.json API should reject empty post (fixes #2698)

nise-nabe/ppcon-sns,openpne/OpenPNE3,openpne/OpenPNE3,tejima/OP3PKG,openpne/OpenPNE3,nise-nabe/ppcon-sns,tejima/OP3PKG,openpne/OpenPNE3