title: init NSData to nil SWIFT
body: <p>How can I init NSData to nil ?</p>
  
  <p>Because later, I need to check if this data is empty before using UIImageJPEGRepresentation.</p>
  
  <p>Something like :</p>
  
  <pre><code>if data == nil {
      data = UIImageJPEGRepresentation(image, 1)
  }
  </code></pre>
  
  <p>I tried data.length == 0 but I don't know why, data.length isn't equal to 0 while I haven't initialized.</p>