title: Logstash conditional check for nil/null value in a field
body: <p>I have a json data with some field value as null (eg: "location": null). I would need to check whether this field is null, and take some action.</p>
  
  <p>I have tried using <code>if [location] == 'null' { do something }</code> but it fails, also I have tried with <code>if [location] == 'nill' { do something }</code></p>
  
  <p>Found some relative links that mentioned to check whether the field exist <code>if [location]</code> but this can't be used in my case.</p>
  
  <p>Please help me to solve this, thanks in advance.</p>