Add null coalesce for label and style attribute

https://github.com/nextcloud/server/pull/16832#discussion_r322380822



Signed-off-by: Jochen <rothjochen@gmail.com>

core/templates/login.php

@@ -8,7 +8,7 @@ script('core', 'dist/login');
 <?php if (!empty($_['alt_login'])) { ?>
     <div id="alternative-logins">
         <?php foreach($_['alt_login'] as $login): ?>
-            <a class="button <?php p(isset($login['style']) ? $login['style'] : ''); ?>" aria-label="<?php p($login['label']); ?>" href="<?php print_unescaped($login['href']); ?>" >
+            <a class="button <?php p($login['style'] ?? ''); ?>" aria-label="<?php p($login['label'] ?? ''); ?>" href="<?php print_unescaped($login['href']); ?>" >
                 <?php p($login['name']); ?>
             </a>
         <?php endforeach; ?>