scheduler: fix NPE when deployment is nil, but placement is a canary

scheduler/generic_sched.go

@@ -517,7 +517,7 @@ func (s *GenericScheduler) computePlacements(destructive, place []placementResul
 
 				// If we are placing a canary and we found a match, add the canary
 				// to the deployment state object and mark it as a canary.
-				if missing.Canary() {
+				if missing.Canary() && s.deployment != nil {
 					if state, ok := s.deployment.TaskGroups[tg.Name]; ok {
 						state.PlacedCanaries = append(state.PlacedCanaries, alloc.ID)
 					}