HTML API: Indicate when WordPress rejects attribute updates.

When setting an an attribute value in the HTML API, WordPress may reject
an update based on rules in `kses`. In these cases, the return value from
an escaping function will be an empty string, and the HTML API should
reject the update. Unfortunately, it currently reports that it updates the
attribute but sets an empty string value, which is misleading.

In this patch, the HTML API will refuse the attribute update and return
false to indicate as much when WordPress rejects the updates.

Developed in WordPress#7114
Discussed in https://core.trac.wordpress.org/ticket/61719

Follow-up to [58472].

Props: amitraj2203, dmsnell, mukesh27.
Fixes #61719.


git-svn-id: https://develop.svn.wordpress.org/trunk@58844 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -3197,6 +3197,12 @@ public function set_attribute( $name, $value ): bool {
 			 * @see https://html.spec.whatwg.org/#attributes-3
 			 */
 			$escaped_new_value = in_array( $comparable_name, wp_kses_uri_attributes() ) ? esc_url( $value ) : esc_attr( $value );
+
+			// If the escaping functions wiped out the update, reject it and indicate it was rejected.
+			if ( '' === $escaped_new_value && '' !== $value ) {
+				return false;
+			}
+
 			$updated_attribute = "{$name}=\"{$escaped_new_value}\"";
 		}