title: Haystack queryset contains None elements
body: <p>I'm using Haystack for search, and the resulting SearchQuerySet returned contains None elements:</p>
  
  <pre><code>&gt;&gt; SearchQuerySet().models(Question, Document, Idea)
  &gt;&gt; [&lt;SearchResult: idea.idea (pk=3875)&gt;, None, None, None]
  </code></pre>
  
  <p>Running rebuild_index doesn't help. If I remove the .models() call from the first line, the problem goes away. I could just filter out None elements from the results, but I wanted to know if this is intended behaviour?</p>
  
  <p>I am using Django 1.4, Whoosh, and Haystack 2.0.0-beta</p>