Fix bug in base 'shifts' template if weekdays not in context

tailbone/templates/shifts/base.mako

@@ -11,11 +11,13 @@
       var data_modified = false;
       var okay_to_leave = true;
       var previous_selections = {};
+      % if weekdays is not Undefined:
       var weekdays = [
           % for i, day in enumerate(weekdays, 1):
               '${day.strftime('%a %d %b %Y')}'${',' if i < len(weekdays) else ''}
           % endfor
       ];
+      % endif
 
       window.onbeforeunload = function() {
           if (! okay_to_leave) {