Widget instance does not return undefined for empty jQuery collection

[Victor Homyakov]

Current versions of jQuery UI return following values for widget instance:

• When widget is initialized: widget instance
• When widget is not initialized: undefined
• When jQuery collection is empty: empty jQuery collection

​https://jsfiddle.net/gtmbwyb9/

This is a bit misleading because as stated in ​https://api.jqueryui.com/jQuery.widget/#method-instance, if the element does not have an associated instance, undefined is returned. I ran into that problem in single-page application, while I was simply checking for non-undefined widget instance before calling widget methods. In case when DOM element is missing, additional check is required.

[Scott González]

In 3dd8a09:

Widget: instance() should return undefined for empty sets

Fixes #15019