title: How to update workitem for a field with no data in TFS
body: <p>I'm using Microsoft.TeamFoundation.WorkItemTracking.Client to fix some stuff.
   The request is: I have workitem with a field which has no data, now I need to update the workitem for that field, how can I make it?</p>
  
  <pre><code>    public void UpdateWorkItem(int workItemId, Dictionary&lt;string, string&gt; values)
      {
          if (workItemId &lt;= 0) return;
  
          var workItem = tfsStore.GetWorkItem(workItemId);
  
          //now if I use workItem.Fields["updatingKey"] it'll throw exception since the field updatingKey does not exist in the Fields of the workitem.
  
      }
  </code></pre>