Check if workflow execution is empty in integrity checker
Check if workflow execution is empty in integrity checker

* Method get_workflow_execution() raises an exception if the workflow
  execution does not exist. Since this is a valid case (the method
  may be called via scheduler after the execution is deleted)
  we need to smoothly handle it.

Change-Id: Ibd6099f1e0fd07c71130f11457b355a367229977

openstack/mistral,openstack/mistral