title: Sort on datagrid also sort null values in flex
body: <p>Today i am getting some weird issue in datagrid.</p>
  
  <pre><code>&lt;s:DataGrid id="grid_result" dataProvider="{somelist}" sortableColumns="true"&gt;
  </code></pre>
  
  <p>I have datagrid and on top header click it sort the column suppose this is the column.</p>
  
  <pre><code>&lt;s:GridColumn headerText="Temp" dataField="tempName"&gt;
  </code></pre>
  
  <p>It will display tempName from someList. But some of data is null. So, when i sort this column on header of this column click it will sort null value also.</p>
  
  <p>like it display:</p>
  
  <pre><code>A
  B
  .
  .
  l
  m
  blank cell
  blank cell
  o
  p
  .
  .
  Z
  </code></pre>
  
  <p>It consider null value after m and before o. I think it consider null as n(I may wrong for this).</p>
  
  <p>I want to display null value at last. I have search lot but not found any solution.</p>
  
  <p>Any help is greatly appreciated.  </p>