Default tardiness_sum to 0 if None

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

@@ -61,7 +61,7 @@
       {% endfor %}
       <tr>
         <th colspan="3">{% trans 'Tardiness' %}</th>
-        <td>{{ person.tardiness_sum }}'/{{ person.tardiness_count }}&times;</td>
+        <td>{{ person.tardiness_sum|default_if_none:0 }}'/{{ person.tardiness_count }}&times;</td>
       </tr>
     </table>
   </div>

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

@@ -43,7 +43,7 @@
       {% for absence_reason in absence_reasons_not_counted %}
         <td>{{ person|get_dict:absence_reason.count_label }}</td>
       {% endfor %}
-      <td>{{ person.tardiness_sum }}'/{{ person.tardiness_count }}&times;</td>
+      <td>{{ person.tardiness_sum|default_if_none:0 }}'/{{ person.tardiness_count }}&times;</td>
       {% for extra_mark in extra_marks %}
         <td>{{ person|get_dict:extra_mark.count_label }}</td>
       {% endfor %}