'Regression test for #12550: display_for_field should handle None
value.'

def test_null_display_for_field(self):

    display_value = display_for_field(None, models.CharField())
    self.assertEqual(display_value, EMPTY_CHANGELIST_VALUE)
    display_value = display_for_field(None, models.CharField(choices=((None, 'test_none'),)))
    self.assertEqual(display_value, 'test_none')
    display_value = display_for_field(None, models.DateField())
    self.assertEqual(display_value, EMPTY_CHANGELIST_VALUE)
    display_value = display_for_field(None, models.TimeField())
    self.assertEqual(display_value, EMPTY_CHANGELIST_VALUE)
    display_value = display_for_field(None, models.NullBooleanField())
    expected = (u'<img    src="%simg/admin/icon-unknown.gif"    alt="None"    />' % settings.ADMIN_MEDIA_PREFIX)
    self.assertEqual(display_value, expected)
    display_value = display_for_field(None, models.DecimalField())
    self.assertEqual(display_value, EMPTY_CHANGELIST_VALUE)
    display_value = display_for_field(None, models.FloatField())
    self.assertEqual(display_value, EMPTY_CHANGELIST_VALUE)
