Fix /NetworkDriver.Join api validation

{
    u'NetworkID': u'45d5a3742767122011568cac8f97364c1320d3d95ed887b7bc45f4a2198fde93',
    u'SandboxKey': u'/var/run/docker/netns/576760c8cec1',
    u'Options': None,
    u'EndpointID': u'5d90c390f684386c4ee8ff5f630fe18f3648632365dd6a80970b7d676486308b'
}

root@vikas-VirtualBox:~# docker run --net=xy -itd --name=conza busybox
075a4395ee9d2d38d471e1fa22bf701ab67a9a4f5e71d544871fed2f46659765
Error response from daemon: Cannot start container 075a4395ee9d2d38d471e1fa22bf701ab67a9a4f5e71d544871fed2f46659765: Plugin Error: NetworkDriver.Join, {
  "Err": "None is not of type u'object'\n\nFailed validating u'type' in schema[u'properties'][u'Options']:\n {u'description': u'Options.', u'example': {}, u'type': u'object'}\n\nOn instance[u'Options']:\n None"