title: FetchXML doesn't return aggregate column when null values encountered
body: <p>I have a FetchXML query that reuturns two aggregate columns:</p>
  
  <pre><code>&lt;fetch distinct='false' mapping='logical' aggregate='true'&gt;
    &lt;entity name='blocktrade'&gt;
      &lt;attribute name='sourceref' alias='trade_count' aggregate='count'/&gt;
      &lt;attribute name='allocationtradecount' alias='alloc' aggregate='sum'/&gt;
      &lt;attribute name='organisation' alias='org' groupby='true'/&gt;
    &lt;/entity&gt;
  &lt;/fetch&gt;
  </code></pre>
  
  <p>If I restrict the query to return objects that have values in allocationtradecount it works as expected. However, if some objects have null for the allocationtradecount, the column is not returned in the results! </p>
  
  <p>i.e.</p>
  
  <pre><code>(int)((AliasedValue)e["alloc_count"]).Value;
  </code></pre>
  
  <p>fails. Is this 'expected'? How can I ensure 0 is used when null values are summed?</p>