title: don't display datetime min value in gridview
body: <p>given:</p>
  
  <pre><code>public class Customer
  {
      public int Id { get; set; }
      public string FirstName { get; set; }
      public DateTime Birthdate { get; set; }
  }
  </code></pre>
  
  <p>I have List where some birthdates weren't entered and so when I bind to GridView it shows DateTime.MinValue. What are some different ways to show a blank in a gridview if the birthdate is DateTime.MinValue?</p>