[TWIG][Templates] check if variable is defined in twig before using it

fixes error in /actor/X, where there's no page_title defined

components/Feed/templates/feed/feed.html.twig

@@ -10,7 +10,9 @@
 
 {% block body %}
     <header class="feed-header">
-        <h1>{{ page_title | trans }} {{ 'feed' | trans }}</h1>
+        {% if page_title is defined %}
+            <h1>{{ page_title | trans }} {{ 'feed' | trans }}</h1>
+        {% endif %}
         <nav class="feed-actions" role=navigation>
             {% for block in handle_event('AddFeedActions', app.request) %}
                 {{ block | raw }}