Fix glitch with undefined disks

appengine/swarming/elements/res/imp/botlist/bot-list-data.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <bot-list-data>
 
@@ skipping 155 matching lines @@
       },
 
       _bots: function(){
         if (!this._list || !this._list.items) {
           return [];
         }
         // Do any preprocessing here
         this._list.items.forEach(function(bot){
           // Parse the state, which is a JSON string.  This contains a lot of
           // interesting information like details about the devices attached.
+          bot.state = bot.state || "{}";
           bot.state = JSON.parse(bot.state);
           // get the disks in an easier to deal with format, sorted by size.
-          var disks = bot.state["disks"];
+          var disks = bot.state.disks || {};
           var keys = Object.keys(disks);
-          if (!keys || !keys.length) {
+          if (!keys.length) {
             bot.disks = [{"id": "unknown", "mb": 0}];
           } else {
             bot.disks = [];
             for (var i = 0; i < keys.length; i++) {
               bot.disks.push({"id":keys[i], "mb":disks[keys[i]].free_mb});
             }
             // Sort these so the biggest disk comes first.
             bot.disks.sort(function(a, b) {
               return b.mb - a.mb;
             });
@@ skipping 87 matching lines @@
 
         // No need to sort any of this, bot-filters sorts secondary items
         // automatically, especially when the user types a query.
         return pMap;
       },
 
     });
   })();
   </script>
 </dom-module>