title: Add attribute in Sightly/HTL only if it exists (AEM)
body: <p>I have a component that takes the path of an image from the dialog and renders it.
  Here's the snippet of the component -</p>
  <pre><code>&lt;div class=&quot;${something}&quot; style=&quot;background-image:url('${properties.backgroundImageReference @ context='styleString'}');&quot;&gt;
  &lt;/div&gt;
  </code></pre>
  <p>This works fine but I would like to add a logic to generate the <code>style</code> attribute only if <code>properties.backgroundImageReference</code> exists.</p>
  <p>Is there a quick way of doing this? Please note that I do not want to use an enclosing <code>data-sly-test</code> condition.</p>
  <p>Thanks</p>