title: Do not insert field if null or empty
body: <p>I have a C# class with some fields and some of them are null. Those that are null I do not want to be inserted into db with null value. I do not want them inserted into db at all.
  How do I achieve that?</p>
  
  <pre><code>class User
  {
      public string FirstName;
      public string LastName;
      public string MidName;
  }
  </code></pre>
  
  <p>Not every user has a MidName, but Mongo inserts into db with null value on MidName field.</p>