Code Modernization: Fix implicitly nullable parameter in WP_HTML_Processor.

PHP 8.4 deprecates implicitly nullable parameters, i.e. typed parameters with a `null` default value, which are not explicitly declared as nullable.

This commit the one instance of this in the `WP_HTML_Processor` class.

Fixed by adding the nullability operator to the type, which is supported since PHP 7.1, so we can use it now the minimum supported PHP version is PHP 7.2.

As this deprecation is thrown at compile time, it can be seen at the top of the test output when running on PHP 8.4 (which will be gone once this change has been committed). It is not possible to write a test to cover this.

Ref: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

Follow-up to [58867], [58769], [58304], [58192].

Props jrf.
See #62061.

git-svn-id: https://develop.svn.wordpress.org/trunk@59053 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

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

@@ -782,7 +782,7 @@ public function matches_breadcrumbs( $breadcrumbs ): bool {
 	 * @return bool|null Whether to expect a closer for the currently-matched node,
 	 *                   or `null` if not matched on any token.
 	 */
-	public function expects_closer( WP_HTML_Token $node = null ): ?bool {
+	public function expects_closer( ?WP_HTML_Token $node = null ): ?bool {
 		$token_name = $node->node_name ?? $this->get_token_name();
 
 		if ( ! isset( $token_name ) ) {