Media: Fix $content parameter default value in img_caption_shortcode().

The shortcode content is expected to be a string, not `null`. `do_shortcode()` expects a string for `$content`.

The `img_caption_shortcode()` also expects a string for the `$content` parameter and is expected to return a string for the HTML content to display the caption. 

Prior to this commit:
The default value for the `$content` parameter was set to `null`. If no `$content` was passed, the function:
- could return `null` when the `$atts['width'] < 1` or there was no caption
- else, it invoked `do_shortcode( $content )` passing `null` which on PHP 8.1+ triggers a deprecation notice:
{{{
strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated
}}}

This commit:

- Fixes the default `$content` value to align to the expected shortcode content of `string`, not `null`.
- Fixes the PHP 8.1 deprecation notice when `null` was being passed to `do_shortcode()`.
- Changes the assertion in a couple of tests to check for the empty string instead of `null.

Follow-up to [8196], [8925], [8239], [26915], [31530], [42704].

Props jrf, hellofromTonya, azaozz, joedolson.
See #53635.
Built from https://develop.svn.wordpress.org/trunk@51816


git-svn-id: http://core.svn.wordpress.org/trunk@51423 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -2085,6 +2085,7 @@ add_shortcode( 'caption', 'img_caption_shortcode' );
  * @since 2.6.0
  * @since 3.9.0 The `class` attribute was added.
  * @since 5.1.0 The `caption_id` attribute was added.
+ * @since 5.9.0 The `$content` parameter default value changed from `null` to `''`.
  *
  * @param array  $attr {
  *     Attributes of the caption shortcode.
@@ -2097,10 +2098,10 @@ add_shortcode( 'caption', 'img_caption_shortcode' );
  *     @type string $caption    The caption text.
  *     @type string $class      Additional class name(s) added to the caption container.
  * }
- * @param string $content Shortcode content.
+ * @param string $content Optional. Shortcode content. Default empty string.
  * @return string HTML content to display the caption.
  */
-function img_caption_shortcode( $attr, $content = null ) {
+function img_caption_shortcode( $attr, $content = '' ) {
 	// 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 = '5.9-alpha-51815';
+$wp_version = '5.9-alpha-51816';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.