Media: Prevent warning if `[caption]` shortcode is used without attributes.

Props maltfield, aslamdoctor.
Fixes #59206.
Built from https://develop.svn.wordpress.org/trunk@56488


git-svn-id: http://core.svn.wordpress.org/trunk@56000 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -2361,6 +2361,10 @@ add_shortcode( 'caption', 'img_caption_shortcode' );
  * @return string HTML content to display the caption.
  */
 function img_caption_shortcode( $attr, $content = '' ) {
+	if ( ! $attr ) {
+		$attr = array();
+	}
+
 	// New-style shortcode with the caption inside the shortcode with the link and image tags.
 	if ( ! isset( $attr['caption'] ) ) {
 		if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is', $content, $matches ) ) {

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-alpha-56484';
+$wp_version = '6.4-alpha-56488';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.