Revert "[Form] Remove "value" attribute on empty_value option"

This reverts commit 9e849eb78bcc668a2b8aad39cb47e2ad32c73a47.

Reasons for the revert:

* symfony/symfony#8478
* symfony/symfony#8526

Author: webmozart

Resources/views/Form/choice_widget_collapsed.html.php

@@ -2,7 +2,7 @@
     <?php echo $view['form']->block($form, 'widget_attributes') ?>
     <?php if ($multiple): ?> multiple="multiple"<?php endif ?>
 >
-    <?php if (null !== $empty_value): ?><option <?php if ($required):?> disabled="disabled"<?php if (empty($value) && "0" !== $value): ?> selected="selected"<?php endif ?><?php else: ?> value=""<?php endif?>><?php echo $view->escape($view['translator']->trans($empty_value, array(), $translation_domain)) ?></option><?php endif; ?>
+    <?php if (null !== $empty_value): ?><option value=""<?php if ($required):?> disabled="disabled"<?php if (empty($value) && "0" !== $value): ?> selected="selected"<?php endif ?><?php endif?>><?php echo $view->escape($view['translator']->trans($empty_value, array(), $translation_domain)) ?></option><?php endif; ?>
     <?php if (count($preferred_choices) > 0): ?>
         <?php echo $view['form']->block($form, 'choice_widget_options', array('choices' => $preferred_choices)) ?>
         <?php if (count($choices) > 0 && null !== $separator): ?>