Add default value for date_of_birth or None is printed

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

@@ -35,7 +35,7 @@
       <td>{{ person.last_name }}</td>
       <td>{{ person.first_name }}</td>
       <td>{{ person.get_sex_display }}</td>
-      <td>{{ person.date_of_birth }}</td>
+      <td>{{ person.date_of_birth|default_if_none:'' }}</td>
       <td>{{ person.absence_count }}</td>
       {% for absence_reason in absence_reasons %}
         <td>{{ person|get_dict:absence_reason.count_label }}</td>