manager: Fix panic when Signer is nil
manager: Fix panic when Signer is nil

The CA key was recently moved inside a Signer object, which may
sometimes be nil. defaultClusterObject is called every time the manager
becomes a leader, even though its return value won't necessarily be
written to Raft. It needs to check if Signer is nil.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

allencloud/swarmkit,deljuven/swarmkit,fate-grand-order/swarmkit,jlhawn/swarmkit,elifa/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,docker/swarmkit,RenaudWasTaken/swarmkit,Noyan759/swarmkit,diogomonica/swarmkit,Noyan759/swarmkit,sungwonh/swarmkit,allencloud/swarmkit,dperny/swarmkit-1,docker/swarmkit,diogomonica/swarmkit,deljuven/swarmkit,jlhawn/swarmkit,cheyang/swarmkit,alyyousuf7/swarmkit,RenaudWasTaken/swarmkit,alyyousuf7/swarmkit,sungwonh/swarmkit,dperny/swarmkit-1,elifa/swarmkit,AliyunContainerService/swarmkit,fate-grand-order/swarmkit