When appRebalanceEnable == true and  rebalanceCb == nil  and channel == nil, we want handle the event by poll , so the rebalance event should return by poll but not nil.
link: 
  
  
    
        
          |  | if channel != nil && c.appRebalanceEnable && c.rebalanceCb == nil { | 
    
   
 
fix example:
	if c.appRebalanceEnable && c.rebalanceCb == nil {
		// Channel-based consumer with rebalancing enabled,
		// return the rebalance event and rely on the  application
		// to call *Assign() / *Unassign().
		return ev
	}