Guard against extra_mark is None

aleksis/apps/alsijil/templates/alsijil/partials/person_overview.html

@@ -129,7 +129,9 @@
         {{ note.documentation.subject.short_name }}
       </td>
       <td>{{ note.documentation.get_teachers_short_names|join:', ' }}</td>
-      <td>{{ note.extra_mark.short_name }}</td>
+      {% if note.extra_mark %}
+        <td>{{ note.extra_mark.short_name }}</td>
+      {% endif %}
       <td>{{ note.note }}</td>
     </tr>
   {% endfor %}