title: HTML&Django-why still get error message for null input for class field models.DateField(null=True)?
body: <p>I have a date data type in one of my class and it is optional, so I set null=True. but when the user input nothing for the date and submit the info, I get the error message of </p>
  
  <blockquote>
    <p>[u"'' value has an invalid date format. It must be in YYYY-MM-DD
    format."]</p>
  </blockquote>
  
  <p>Field :<code>birthday=models.DateField(null=True)</code></p>
  
  <p>HTML code is as below.</p>
  
  <p><code>&lt;div&gt;
              &lt;label&gt;Birthday:&lt;/label&gt;
              &lt;input type="date" id="id_bd" name="bd"&gt;
          &lt;/div&gt;</code></p>
  
  <p>Can anyone explain to me why this would happen and how am I going to handle this null input error? Thank you very much.</p>