handle undefined when sorting quick help

since undefined is neither less than nor greater than
anything in Javascript,
the sort function was treating it as equal to everything, causing inconsistent behavior,
depending on the sort algorithm of the browser.

This ensures undefined elements are sorted last in the sequence.

IPython/html/static/base/js/keyboard.js

@@ -244,13 +244,13 b' define(['
             }
         }
         help.sort(function (a, b) {
-            if (a.help_index > b.help_index){
-                return 1;
+            if (a.help_index === b.help_index) {
+                return 0;
             }
-            if (a.help_index < b.help_index){
-                return -1;
+            if (a.help_index === undefined || a.help_index > b.help_index){
+                return 1;
             }
-            return 0;
+            return -1;
         });
         return help;
     };