Form #states does not work on a "checkboxes" element

Simplify a little because drupal_render() does this:

  // Add any JavaScript state information associated with the element.
  if (!empty($elements['#states'])) {
    drupal_process_states($elements);
  }

So we can put NULL in '#states' and that will not be processed.

I use !empty($elements['#states']) instead of isset($elements['#states']) like the '#ajax' key becuase drupal_render() uses !empty().

You won't see anything different with this patch unless you use the '#states' on a "checkboxes" form element like this:

  $form['fruits'] = array(
    '#type' => 'checkboxes',
    '#options' => drupal_map_assoc(array(t('Apple'), t('Banana'), t('Peach'), t('Orange'))),
    '#title' => t('What do you want to eat?'),
    '#states' => array(
      'checked' => array(
        ':input[name="eat_all"]' => array('checked' => TRUE),
      ),
    ),
  );

  $form['eat_all'] = array(
    '#type' => 'checkbox',
    '#title' => t('Eat all'),
  );

Before the patch, clicking on the "Eat all" checkbox does nothing to the "fruits" checkboxes. With the patch, all the checkboxes in "fruits" follow the check state of "eat_all".