MDL-60574 atto_undo: fixed redo erasing content

When you'd hit ctrl-y with nothing on the redo stack the entire
content of Atto would get deleted with no way of getting it back.

lib/editor/atto/plugins/undo/yui/build/moodle-atto_undo-button/moodle-atto_undo-button-debug.js

@@ -257,6 +257,12 @@ Y.namespace('M.atto_undo').Button = Y.Base.create('button', Y.M.editor_atto.Edit
      */
     _redoHandler: function(e) {
         e.preventDefault();
+
+        // Don't do anything if redo stack is empty.
+        if (this._redoStack.length === 0) {
+            return;
+        }
+
         var html = this._getHTML(),
             redo = this._getRedo();
 

lib/editor/atto/plugins/undo/yui/build/moodle-atto_undo-button/moodle-atto_undo-button.js

@@ -257,6 +257,12 @@ Y.namespace('M.atto_undo').Button = Y.Base.create('button', Y.M.editor_atto.Edit
      */
     _redoHandler: function(e) {
         e.preventDefault();
+
+        // Don't do anything if redo stack is empty.
+        if (this._redoStack.length === 0) {
+            return;
+        }
+
         var html = this._getHTML(),
             redo = this._getRedo();
 

lib/editor/atto/plugins/undo/yui/src/button/js/button.js

@@ -255,6 +255,12 @@ Y.namespace('M.atto_undo').Button = Y.Base.create('button', Y.M.editor_atto.Edit
      */
     _redoHandler: function(e) {
         e.preventDefault();
+
+        // Don't do anything if redo stack is empty.
+        if (this._redoStack.length === 0) {
+            return;
+        }
+
         var html = this._getHTML(),
             redo = this._getRedo();