[ticket/12604] Fix wrong padding when no notifications

Added a class to the <li> when the <li> represents that there's no
notifications (no_notifications) and
Added a CSS rule that matches the change to the HTML source for when
there's no notifications.

PHPBB3-12604
Merge pull request #2506 from brunoais/ticket/12604

[ticket/12604] Fix wrong padding when no notifications

* brunoais/ticket/12604:
  [ticket/12604] Fix wrong padding when no notifications