Be a little more liberal with what count's as not having a due date in a section

cms/templates/edit_subsection.html

@@ -80,7 +80,7 @@
         <p class="date-description">
           <%
               # due date uses it own formatting for stringifying the date. As with capa_module.py, there's a utility module available for us to use
-             due_date = dateutil.parser.parse(subsection.lms.due) if subsection.lms.due is not None else None
+             due_date = dateutil.parser.parse(subsection.lms.due) if subsection.lms.due else None
           %>
           <input type="text" id="due_date" name="due_date" value="${due_date.strftime('%m/%d/%Y') if due_date is not None else ''}" placeholder="MM/DD/YYYY" class="date" size='15' autocomplete="off"/>
           <input type="text" id="due_time" name="due_time" value="${due_date.strftime('%H:%M') if due_date is not None else ''}" placeholder="HH:MM" class="time" size='10' autocomplete="off"/>