Show a blank icon in PrefFeedTree if there is no favicon for a feed.

Duplicates behavior of FeedTree.
Prevents a broken request to 'false' if a feed doesn't have a favicon.

js/PrefFeedTree.js

@@ -23,12 +23,14 @@ require(["dojo/_base/declare", "dojo/dom-construct", "lib/CheckBoxTree"], functi
 		_createTreeNode: function(args) {
 			var tnode = this.inherited(arguments);
 
-			if (args.item.icon) {
 			var icon = dojo.doc.createElement('img');
+			if (args.item.icon && args.item.icon[0]) {
 				icon.src = args.item.icon[0];
+			} else {
+				icon.src = 'images/blank_icon.gif';
+			}
 			icon.className = 'tinyFeedIcon';
 			domConstruct.place(icon, tnode.iconNode, 'only');
-			}
 
 			var param = this.model.store.getValue(args.item, 'param');