title: `Skip on empty` not working in Yii2 file upload
body: <p>I have  a provision to upload logo for companies in my application. Uploading and saving on creating profile works fine. But on update, logo goes empty if I am not uploading it again!</p>
  
  <p>Here's my update form</p>
  
  <pre><code>    &lt;?php $form = ActiveForm::begin([
                  'options' =&gt; ['enctype'=&gt;'multipart/form-data']
          ]); ?&gt;
  
         .....
  
         &lt;?= $form-&gt;field($model, 'logo')-&gt;fileInput() ?&gt;
  
         ...
  </code></pre>
  
  <p>My Controller action</p>
  
  <pre><code>    if ($model-&gt;load($_POST) ) {
      $file = \yii\web\UploadedFile::getInstance($model, 'logo');
      if($file){
      $model-&gt;logo=$file; }
  
      if($model-&gt;save()){
  
          if($file)
          $file-&gt;saveAs(\Yii::$app-&gt;basePath . '/web/images/'.$file);
          }
  
              return $this-&gt;redirect(['profile']);
          } else {
              return $this-&gt;renderPartial('update', [
                  'model' =&gt; $model,
              ]);
          }
  </code></pre>
  
  <p>My Rules:</p>
  
  <pre><code>public function rules()
  {
      return [
  
          [['logo'], 'image', 'extensions' =&gt; 'jpg,png', 'skipOnEmpty' =&gt; true],
          [['name'], 'required'],
          [['name', 'description'], 'string'],
  
      ];
  }
  </code></pre>
  
  <p>Any ideas????</p>