grabHelper: Make sure to return not null when the stack contains fake grabs

We did this if there's nothing in the grab stack, but didn't do this
when there's no real grabs in the grab stack.

js/ui/grabHelper.js

@@ -84,14 +84,11 @@ const GrabHelper = new Lang.Class({
     },
 
     get currentGrab() {
-        if (!this._grabStack.length)
-            return {};
-
         let idx = this._grabStack.length - 1;
         while (idx >= 0 && this._grabStack[idx].untracked)
             idx--;
 
-        return this._grabStack[idx];
+        return this._grabStack[idx] || {};
     },
 
     _findStackIndex: function(actor) {