Widgets: Don't try mapping empty sidebars.

Fixes a bug where the mapping logic would try mapping empty sidebars, resulting in PHP warnings.

Props ionvv, chetan200891 for initial patch.
Fixes #42603.


Built from https://develop.svn.wordpress.org/trunk@42362


git-svn-id: http://core.svn.wordpress.org/trunk@42191 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42361';
+$wp_version = '5.0-alpha-42362';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/widgets.php

@@ -1333,6 +1333,9 @@ function wp_map_sidebars_widgets( $existing_sidebars_widgets ) {
 
 	if ( is_array( $old_sidebars_widgets ) ) {
 
+		// Remove empty sidebars, no need to map those.
+		$old_sidebars_widgets = array_filter( $old_sidebars_widgets );
+
 		// Only check sidebars that are empty or have not been mapped to yet.
 		foreach ( $new_sidebars_widgets as $new_sidebar => $new_widgets ) {
 			if ( array_key_exists( $new_sidebar, $old_sidebars_widgets ) && ! empty( $new_widgets ) ) {