Do not close nil channel. With the change in swarm, engine in pending state may have a dockerclient where monitor channel is not created.
Do not close nil channel. With the change in swarm, engine in pending state may have a dockerclient where monitor channel is not created.

Signed-off-by: Dong Chen <252dc6b7229d6040aded2d6fd307bed15f92f3c6@docker.com>

samalba/dockerclient,denverdino/dockerclient,BlueDragonX/dockerclient,ehazlett/dockerclient