[ticket/13040] Fix W3C validator warning in overall_footer.html

Added a non-breaking space to remove the following warning:
**Empty heading.**

```html
<h3 class="alert_title"></h3><p class="alert_text"></p>
```

PHPBB3-13040
Merge remote-tracking branch 'prototech/ticket/13040' into develop-ascraeus

* prototech/ticket/13040:
  [ticket/13040] Fix W3C validator warning in overall_footer.html