Ignore missing matomo_custom_title in tests

The title is only set for specific page, by default it’s derived from
the page title. Otherwise, tests fail locally with:

```
Failed: Undefined template variable 'matomo_custom_title' in 'itou/templates/layout/base.html'
```

itou/templates/layout/base.html

@@ -231,7 +231,7 @@
                     window._paq.push(['setCustomUrl', new URL('{{ matomo_custom_url }}', window.location.origin).href]);
                 {% endif %}
 
-                {% if matomo_custom_title %}
+                {% if matomo_custom_title|default:"" %}
                     window._paq.push(['setDocumentTitle', '{{ matomo_custom_title }}']);
                 {% else %}
                     /* defaults to a stripped version (no suffix) of the page title. */