title: Cast to nullable list of double gives an error
body: <p>In my database I have DOUBLE PRECESION type(Postgressql)
  in my query the field which is double precision could be null ,so I have made this list nullable  but during the casting I get </p>
  
  <blockquote>
    <p>"Specified cast is not valid"</p>
  </blockquote>
  
  <p>List is defined like this:</p>
  
  <pre><code>public List&lt;double?&gt; abb_conv { get; set; }
  </code></pre>
  
  <p>I got error in this line:</p>
  
  <pre><code>  _powerConv.abb_conv.Add((double?)dt["abb_conv"]);
  </code></pre>
  
  <p>where <code>dt</code> is the name of <code>Datatable</code></p>