title: angular-ui-select with empty <option> shows 2 empty fields
body: <p>I am using <code>angular-ui-select</code> for dropdowns. I need to have dropdown that you can cancel.</p>
  
  <p>Here is my dropdown in template:</p>
  
  <pre><code>&lt;select ui-select2="select2Options" name="Machine" ng-model="selectedMachine.type" data-placeholder="type" required&gt;
      &lt;option value=""&gt;&lt;/option&gt;
      &lt;option ng-repeat="type in machineTypes" value="{{type}}"&gt;{{type}}&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Here is <code>select2Options</code>:</p>
  
  <pre><code>$rootScope.select2Options = {
    minimumResultsForSearch: 5,
    allowClear: true,
    ...
  }
  </code></pre>
  
  <p>Here is the result:<br>
  <a href="https://i.stack.imgur.com/Fatgc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Fatgc.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/LAImM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LAImM.png" alt="enter image description here"></a></p>
  
  <p>As you can see, there is a cancel button (and it works), but something weird happens on the picture to the left. <strong>Cause of the <code>&lt;option value=""&gt;&lt;/option&gt;</code> there are two empty fields that you can check.</strong></p>
  
  <p>If you delete <code>&lt;option value=""&gt;&lt;/option&gt;</code>, weird empty fields disappear, however there is no cancel button either:<br>
  <a href="https://i.stack.imgur.com/9G5nm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9G5nm.png" alt="enter image description here"></a></p>
  
  <p><strong>What should I do to have a cancel button and don't have empty fields?</strong></p>
  
  <p><em>angular: v1.4.8<br>
  angular-ui-select2: v0.0.5<br>
  select2: v3.4</em></p>