Customize: Correct the check for rendered widgets in `WP_Customize_Widgets::is_widget_rendered()`.

Make the structure of `::$rendered_sidebars` and `::$rendered_widgets` properties consistent.

This resolves an issue with every widget being marked as inactive by default on the Widgets panel.

Props dlh, afercia, SergeyBiryukov.
Fixes #50508.
Built from https://develop.svn.wordpress.org/trunk@48299


git-svn-id: https://core.svn.wordpress.org/trunk@48068 1a063a9b-81f0-0310-95a4-ce76da25c4cd