Fix fabric.util.enlivenObjects for empty fabric.Group objects

Add unit test
Fixes issue #943

src/util/misc.js

@@ -174,12 +174,11 @@
    * @param {Function} [reviver] Method for further parsing of object elements, called after each fabric object created.
    */
   function enlivenObjects(objects, callback, namespace, reviver) {
+    objects = objects || [ ];
 
     function onLoaded() {
       if (++numLoadedObjects === numTotalObjects) {
-        if (callback) {
-          callback(enlivenedObjects);
-        }
+        callback && callback(enlivenedObjects);
       }
     }
 
@@ -187,6 +186,11 @@
         numLoadedObjects = 0,
         numTotalObjects = objects.length;
 
+    if (!numTotalObjects) {
+      callback && callback(enlivenedObjects);
+      return;
+    }
+
     objects.forEach(function (o, index) {
       // if sparse array
       if (!o || !o.type) {

test/unit/canvas.js

@@ -416,6 +416,26 @@
     }, 500);
   });
 
+  asyncTest('loadFromJSON with empty fabric.Group', function() {
+    var c1 = new fabric.Canvas('c1'),
+        c2 = new fabric.Canvas('c2'),
+        group = new fabric.Group();
+
+    c1.add(group);
+    ok(!c1.isEmpty(), 'canvas is not empty');
+
+    var json = c1.toJSON();
+    var fired = false;
+    c2.loadFromJSON(json, function() {
+      fired = true;
+    });
+
+    setTimeout(function() {
+      ok(fired, 'Callback should be fired even if empty fabric.Group exists');
+      start();
+    }, 500);
+  });
+
   asyncTest('loadFromJSON with async content', function() {
     var group = new fabric.Group([
       new fabric.Rect({ width: 10, height: 20 }),