Menu: Update classes option to use empty string instead of null

Author: arschmitz

tests/unit/menu/menu_common.js

@@ -2,10 +2,10 @@ TestHelpers.commonWidgetTests( "menu", {
 	defaults: {
 		classes: {
 			"ui-menu": "ui-corner-all",
-			"ui-menu-icons": null,
-			"ui-menu-icon": null,
-			"ui-menu-item": null,
-			"ui-menu-divider": null
+			"ui-menu-icons": "",
+			"ui-menu-icon": "",
+			"ui-menu-item": "",
+			"ui-menu-divider": ""
 		},
 		disabled: false,
 		icons: {

ui/menu.js

@@ -32,10 +32,10 @@ return $.widget( "ui.menu", {
 	options: {
 		classes: {
 			"ui-menu": "ui-corner-all",
-			"ui-menu-icons": null,
-			"ui-menu-icon": null,
-			"ui-menu-item": null,
-			"ui-menu-divider": null
+			"ui-menu-icons": "",
+			"ui-menu-icon": "",
+			"ui-menu-item": "",
+			"ui-menu-divider": ""
 		},
 		icons: {
 			submenu: "ui-icon-caret-1-e"