title: Cannot compare DbGeometry with null value to DbNull even when exception throw DbNull as type
body: <p>As in title,
  I have a code, where I recieve DbNull type with null value, when there is no geometry in database and DbGeometry get null value. But when I want to make an if statement to check if DbGeometry equals to DbNull.Value I can't do that.
  It's something like this:</p>
  
  <pre><code>var geom = entity.Select(entity =&gt; entity.Geom);
  if (geom == DbNull.Value) return null; // Here it is not valid for VisualStudio
  </code></pre>
  
  <p>But when i do something like that:</p>
  
  <pre><code>if (geom == null) return null;
  </code></pre>
  
  <p>It works just fine. Why I get an error when I try to compare geom variable which is a DbGeometry type with DbNull.Value even though without handling a null I get a DbNull type.</p>