title: Seyren trigger alert if value is null
body: <p>I try to trigger a Seyren alert if the value received is null, but can't find a way to do so. </p>
  
  <p>I have read several answers, like <a href="https://github.com/scobal/seyren/issues/136" rel="nofollow noreferrer">here</a> and also <a href="https://stackoverflow.com/questions/26775439/how-to-alert-in-seyren-with-graphite-if-transactions-in-last-60-minutes-are-less">here</a> which says that it should be possible with the <code>Allow no data</code> checkbox. </p>
  
  <p>However, what should I put into the field <code>Warn level</code> and <code>Error level</code>, since those fields are useless though mandatory?</p>
  
  <p>Thanks for your help!</p>