fix lastmodifiedexpression

For example if table {{post}} empty :

```php
public function filters()
{
    return array(
        array(
            'CHttpCacheFilter + index',
            'lastModified'=>Yii::app()->db->createCommand("SELECT MAX(`update_time`) FROM {{post}}")->queryScalar(),
            'lastModifiedExpression'=>'Yii::app()->db->createCommand("SELECT MAX(`update_time`) FROM {{post}}")->queryScalar()',
        ),
    );
}
```

if i use only lastModified is ok.
If i use lastModifiedExpression i catch exception