title: Check for nil with guard instead of if?
body: <p>Is there a <code>guard</code> equivalent of checking if a variable is <code>nil</code>? If so how would I translate a statement like this to use <code>guard</code> instead?</p>
  
  <pre><code>if post["preview"]! != nil {
      //do stuff
  } else {
      //handle case where the variable is nil
  }
  </code></pre>