Transition issue when it is moved to backlog (sprint is empty) with Listener

[Анна Рыбина]

I'm trying to write a Scriptrunner listener that transition an issue to status named "Backlog" when it's moved to Backlog at board. So Sprint field value is empty

Event: Issue updated

Condition: 

I've tried several, including simple

cfValues['Sprint'] == null

or

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.greenhopper.service.sprint.Sprint
import com.atlassian.jira.component.ComponentAccessor;
import com.atlassian.jira.issue.CustomFieldManager;
import com.atlassian.jira.issue.Issue;
import com.atlassian.jira.issue.fields.CustomField;
import com.atlassian.greenhopper.service.rapid.view.RapidViewService
import com.atlassian.greenhopper.service.sprint.SprintIssueService
import com.atlassian.greenhopper.service.sprint.SprintManager
import com.atlassian.greenhopper.service.sprint.SprintService

CustomFieldManager customFieldManager = ComponentAccessor.getCustomFieldManager()
CustomField myCustomField = customFieldManager.getCustomFieldObjectByName("Sprint")

sprints = event.issue.getCustomFieldValue(myCustomField)

def isSprint = false
sprints.each{

if(sprints == null){
isSprint = true
}
}

return isSprint

But nothing works. isSprint value is always false on logs

Action: transition to Backlog status