title: How to query nullable boolean records in LLBL?
body: <p>I got into a tricky issue. 
  I am trying to query some records, and the data type is nullable Boolean.
  Now, I want to query these records based on their values: true, false, null. 
  I know how to do true and false, but have no idea how to filter records are null. Can someone help me? </p>
  
  <p>Here is code, the System.DBNull.Value doesn't work. </p>
  
  <pre><code>filter.AddWithOr(VwFields.Discontinued == false);
  filter.AddWithOr(VwFields.Discontinued == true);
  filter.AddWithOr(VwFields.Discontinued == System.DBNull.Value); //wrong
  </code></pre>