title: store null as default value againt Null boolean field
body: <p>How to use <code>NullBooleanField</code> in <code>Django</code> models with <code>SQLlite3</code>,
  I want to store <code>null</code> as default value for the <code>nullBoolean</code> field, how do I do that?</p>
  
  <p>I have migrated db from <code>BooleanField</code> to <code>NullBooleanField</code>, but it is still storing False as the default value. </p>
  
  <p>Usage in the code is as:</p>
  
  <pre><code>LOCATOR_YES_NO_CHOICES = ((None,''), (True,'Yes'), (False, 'No'))
  employed = models.NullBooleanField(choices=LOCATOR_YES_NO_CHOICES,
                                  max_length=3,
                                  blank=True, null=True, default="",)
  </code></pre>
  
  <p>Any example would be of great help.</p>