Disable sequence nav buttons when the sequence is empty

common/lib/xmodule/xmodule/js/src/sequence/display.coffee

@@ -65,6 +65,11 @@ class @Sequence
   toggleArrows: =>
     @$('.sequence-nav-buttons a').unbind('click')
 
+    if @contents.length == 0
+      @$('.sequence-nav-buttons .prev a').addClass('disabled')
+      @$('.sequence-nav-buttons .next a').addClass('disabled')
+      return
+
     if @position == 1
       @$('.sequence-nav-buttons .prev a').addClass('disabled')
     else