title: jquery ui cannot add li to sortable list after all li dragged out or removed
body: <p>I have a sortable set up like so:</p>
  
  <pre><code>            &lt;ul id="sortable2" class="connectedSortable"&gt;
                  &lt;li class="ui-state-default"&gt;First &lt;a title='delete' class="itemDelete text-right"&gt;&lt;i class="fa fa-minus-circle" aria-hidden="true"&gt;&lt;/i&gt;&lt;/a&gt;&lt;/li&gt;
                  &lt;li class="ui-state-default"&gt;Second &lt;a title='delete' class="itemDelete"&gt;&lt;i class="fa fa-minus-circle" aria-hidden="true"&gt;&lt;/i&gt;&lt;/a&gt;&lt;/li&gt;
                  &lt;li class="ui-state-default"&gt;Third &lt;a title='delete' class="itemDelete"&gt;&lt;i class="fa fa-minus-circle" aria-hidden="true"&gt;&lt;/i&gt;&lt;/a&gt;&lt;/li&gt;
              &lt;/ul&gt;
  </code></pre>
  
  <p>When I have dragged all the child elements, the <code>li</code>'s out of the <code>ul</code> I cannot add anymore since there needs to be one <code>li</code> in place for me to add. How can I make it so that even when it is empty I can still drag from sortable1 into this sortable2.</p>