collapsibleTabs: Don't mix Arrays and jQuery objects for one property

We treat this as a jQuery object throughout the code, but it was
initialized to an array.

Bug: T109259
Change-Id: I44c10683ded05656eb7ca1b4be43ddc4f9281076

collapsibleTabs.js

@@ -14,8 +14,7 @@
 		this.each( function () {
 			var $el = $( this );
 			// add the element to our array of collapsible managers
-			$.collapsibleTabs.instances = ( $.collapsibleTabs.instances.length === 0 ?
-				$el : $.collapsibleTabs.instances.add( $el ) );
+			$.collapsibleTabs.instances = $.collapsibleTabs.instances.add( $el );
 			// attach the settings to the elements
 			$el.data( 'collapsibleTabsSettings', settings );
 			// attach data to our collapsible elements
@@ -37,7 +36,7 @@
 		return this;
 	};
 	$.collapsibleTabs = {
-		instances: [],
+		instances: $( [] ),
 		boundEvent: null,
 		defaults: {
 			expandedContainer: '#p-views ul',