Customize: Handle filtering `sidebars_widgets` when the underlying option is non-existent.

Fixes warning related to a non-array argument for `array_merge()` in `WP_Customize_Widgets::customize_register()`. 

Merge of [37352] to the 4.5 branch.

See [37166].
See #36389.
Fixes #36660.
Built from https://develop.svn.wordpress.org/branches/4.5@37453


git-svn-id: http://core.svn.wordpress.org/branches/4.5@37421 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-widgets.php

@@ -1061,7 +1061,7 @@ final class WP_Customize_Widgets {
 	 * @return array
 	 */
 	public function preview_sidebars_widgets( $sidebars_widgets ) {
-		$sidebars_widgets = get_option( 'sidebars_widgets' );
+		$sidebars_widgets = get_option( 'sidebars_widgets', array() );
 
 		unset( $sidebars_widgets['array_version'] );
 		return $sidebars_widgets;

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.5.3-alpha-37451';
+$wp_version = '4.5.3-alpha-37453';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.