title: dataSet.writeXml does not write an empty tag if the field was empty C#
body: <p>I want to create a XML of my DataSet using the .writeXml-function.
  If an entry in my DataSet was null, i would like to have an empty tag instead of a missing tag:</p>
  
  <p>example how it should look like if John had no age:</p>
  
  <pre><code>  &lt;MyDataSet&gt;
      &lt;ID&gt;8613458&lt;/ID&gt;
      &lt;AW_ID&gt;37534778&lt;/AW_ID&gt;
      &lt;NAME&gt;Peter&lt;/NAME&gt;
      &lt;AGE&gt;22&lt;/AGE&gt;
    &lt;/MyDataSet&gt;
    &lt;MyDataSet&gt;
      &lt;ID&gt;8613459&lt;/ID&gt;
      &lt;AW_ID&gt;37534779&lt;/AW_ID&gt;
      &lt;NAME&gt;John&lt;/NAME&gt;
      &lt;AGE&gt;&lt;/AGE&gt;
    &lt;/MyDataSet&gt;
  </code></pre>
  
  <p>example how it looks like right now:</p>
  
  <pre><code>  &lt;MyDataSet&gt;
      &lt;ID&gt;8613458&lt;/ID&gt;
      &lt;AW_ID&gt;37534778&lt;/AW_ID&gt;
      &lt;NAME&gt;Peter&lt;/NAME&gt;
      &lt;AGE&gt;22&lt;/AGE&gt;
    &lt;/MyDataSet&gt;
    &lt;MyDataSet&gt;
      &lt;ID&gt;8613459&lt;/ID&gt;
      &lt;AW_ID&gt;37534779&lt;/AW_ID&gt;
      &lt;NAME&gt;John&lt;/NAME&gt;
    &lt;/MyDataSet&gt;
  </code></pre>
  
  <p>Do you know what I need to do?
  Thanks for your help!!</p>