title: How to add an empty item to ExtJS combobox?
body: <p>I want to add and empty item (display value is blank, item height is kept as normal) to an Ext.form.ComboBox. I refered 2 links below to configure my combobox, but it still not display the empty item:</p>
  <ul>
  <li><a href="http://www.ashlux.com/wordpress/2009/09/04/handling-empty-options-with-ext-js-combo-box/" rel="noreferrer">http://www.ashlux.com/wordpress/2009/09/04/handling-empty-options-with-ext-js-combo-box/</a></li>
  <li><a href="http://www.sencha.com/forum/showthread.php?52340-How-to-add-a-quot-blank-quot-entry-to-a-ComboBox" rel="noreferrer">http://www.sencha.com/forum/showthread.php?52340-How-to-add-a-quot-blank-quot-entry-to-a-ComboBox</a></li>
  </ul>
  <p>Here is my code:</p>
  <pre><code>this.abcCombo = new Ext.form.ComboBox({
      name : 'abcCombo',
      hiddenName : 'abcCombo-id',
      fieldLabel : &quot;My Combobox&quot;,
      width : 250,
      editable : false,
      forceSelection : true,
      mode : 'local',
      triggerAction : 'all',
      valueField : 'id',
      displayField : 'fullName',
      store : new Ext.data.JsonStore({
          fields : ['id', 'fullName']
      }),
      tpl : '&lt;tpl for=&quot;.&quot;&gt;&lt;div class=&quot;x-combo-list-item&quot;&gt;{fullName}&amp;nbsp;&lt;/div&gt;&lt;/tpl&gt;'
  });
  </code></pre>
  <p>The combobox store's data will be loaded after an Ajax request (i.e 3 items in data items). And the combobox has only 3 item (not 4 as I expected).
  Do you have any idea about my problem?!
  Thank you so much!</p>