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

Author: arschmitz

tests/unit/tabs/tabs_common.js

@@ -3,13 +3,13 @@ TestHelpers.commonWidgetTests( "tabs", {
 		active: null,
 		classes: {
 			"ui-tabs": "ui-corner-all",
-			"ui-tabs-collapsible": null,
-			"ui-tabs-active": null,
+			"ui-tabs-collapsible": "",
+			"ui-tabs-active": "",
 			"ui-tabs-nav": "ui-corner-all",
 			"ui-tab": "ui-corner-top",
-			"ui-tabs-anchor": null,
+			"ui-tabs-anchor": "",
 			"ui-tabs-panel": "ui-corner-bottom",
-			"ui-tabs-loading": null
+			"ui-tabs-loading": ""
 		},
 		collapsible: false,
 		disabled: false,

ui/tabs.js

@@ -31,13 +31,13 @@ return $.widget( "ui.tabs", {
 		active: null,
 		classes: {
 			"ui-tabs": "ui-corner-all",
-			"ui-tabs-collapsible": null,
-			"ui-tabs-active": null,
+			"ui-tabs-collapsible": "",
+			"ui-tabs-active": "",
 			"ui-tabs-nav": "ui-corner-all",
 			"ui-tab": "ui-corner-top",
-			"ui-tabs-anchor": null,
+			"ui-tabs-anchor": "",
 			"ui-tabs-panel": "ui-corner-bottom",
-			"ui-tabs-loading": null
+			"ui-tabs-loading": ""
 		},
 		collapsible: false,
 		event: "click",