title: How to check whether ng-content exists
body: <p>Suppose i have simple Bootstrap panel component with multiple transclusion slots. Template example:</p>
  
  <pre><code>&lt;div class="panel panel-default"&gt;
    &lt;div class="panel-heading"&gt;
      &lt;ng-content select="my-panel-heading"&gt;&lt;/ng-content&gt;
    &lt;/div&gt;
    &lt;div class="panel-body"&gt;
      &lt;ng-content select="my-panel-content"&gt;&lt;/ng-content&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I want to make panel-heading optional. How do i hide <code>&lt;div class="panel-heading"&gt;</code> element, if there is no content provided for <code>&lt;ng-content select="my-panel-heading"&gt;&lt;/ng-content&gt;</code></p>