title: I can't compare sql_variant field with IS NULL operator
body: <p>I mean I can't query null values when column type <code>sql_variant</code></p>
  
  <p>For example <code>docsdate</code> table look like this:</p>
  
  <pre><code>ValID DocID     Value  &lt;--sql variant column)
  
   1. 488 146      30.10.2007
   2. 740 190      31.03.2008
   3. 570 161      31.10.2008 
   4. 242 103      NULL
   5. 248 104      NULL
  </code></pre>
  
  <p>When query like <code>select * from docsdate where value is null</code> 
  no rows returned</p>
  
  <p>Any idea?</p>