Customizer undefined property if widget area before/after values are empty

[Clorith]

Introduced in r37322 the replace on widgetClasses casts a TypeError if all the before_widget, after_widget, before_title and after_title fields are declared as an empty string in a theme when registering a widget area through register_sidebar

A quick example registration;

<?php
register_sidebar( array(
        'name'          => __( 'Sidebar' ),
        'id'            => 'sidebar-1',
        'before_widget' => '',
        'after_widget'  => '',
        'before_title'  => '',
        'after_title'   => '',
) );

As reported by @stevengliebe in ​https://wordpress.org/support/topic/customizer-js-error-when-register_sidebar-beforeafter-arguments-empty

[westonruter]

@stevengliebe Please give 37478.0.diff​ a try.

[stevengliebe]

That solves it for me. Thanks for working on this.

[westonruter]

In 38166:

Customize: Prevent customize-preview-widgets JS errors in preview if a sidebar is registered with empty before_widget/after_widget params.

Selective refresh will not be available for widgets when they lack these params, so previewing will fallback to full page refreshes. Sidebars registered as such should be rare so this accounts for an edge case.

Fixes #37478.