Users: Set correct default value for $user_login in retrieve_password().

This resolves a "passing null to non-nullable" deprecation notice on PHP 8.1+:
{{{
Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated
}}}

Follow-up to [50129], [54477].

Props afragen, peterwilsoncc, SergeyBiryukov.
Fixes #62298.
Built from https://develop.svn.wordpress.org/trunk@59312


git-svn-id: http://core.svn.wordpress.org/trunk@58698 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -3106,7 +3106,7 @@ function check_password_reset_key( $key, $login ) {
  *                           Defaults to `$_POST['user_login']` if not set.
  * @return true|WP_Error True when finished, WP_Error object on error.
  */
-function retrieve_password( $user_login = null ) {
+function retrieve_password( $user_login = '' ) {
 	$errors    = new WP_Error();
 	$user_data = false;
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.8-alpha-59307';
+$wp_version = '6.8-alpha-59312';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.