objectManager: handle object manager sending empty array

If the service implementing the object manager returns
a 0 length array we currently throw an exception.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=780403

js/misc/objectManager.js

@@ -193,6 +193,11 @@ const ObjectManager = new Lang.Class({
 
             let [objects] = result;
 
+            if (!objects) {
+                this._tryToCompleteLoad();
+                return;
+            }
+
             let objectPaths = Object.keys(objects);
             for (let i = 0; i < objectPaths.length; i++) {
                 let objectPath = objectPaths[i];