Avoid checking if slice is nil
Avoid checking if slice is nil

It's more correct to check if len(slice) == 0. In the current code, this
doesn't make a difference, but the nil check was fragile.

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

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