Ignore nil events from go-dockerclient
Ignore nil events from go-dockerclient

With the recent update of github.com/fsouza/go-dockerclient, suddenly
seeing nil events sent to our channel.  Ignore those.

control-center/serviced,control-center/serviced,control-center/serviced,control-center/serviced,control-center/serviced,control-center/serviced,control-center/serviced,control-center/serviced