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

Props maltfield, aslamdoctor.
Fixes #59206.

git-svn-id: https://develop.svn.wordpress.org/trunk@56488 602fd350-edb4-49c9-b593-d223f7449a82

Author: swissspidy

src/wp-includes/media.php

@@ -2361,6 +2361,10 @@ function _wp_post_thumbnail_context_filter_remove() {
  * @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 ) ) {