Improve the looks of the previous commit by using an infinity sign in

case of a missing start or end date
 - Legacy-Id: 12162

ietf/templates/review/unavailable_table.html

@@ -1,7 +1,7 @@
 <table class="simple-table">
   {% for p in unavailable_periods %}
     <tr class="unavailable-period-{{ p.state }}">
-      <td>{{ p.start_date|default:"" }} - {{ p.end_date|default:"" }}</td>
+      <td> {% if p.start_date or p.end_date %}{{ p.start_date|default:"&infin;" }} -{% endif %} {{ p.end_date|default:"&infin;" }}</td>
       <td>{{ p.get_availability_display }}</td>
     </tr>
   {% endfor %}