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.

git-svn-id: https://develop.svn.wordpress.org/trunk@59312 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/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;
 

tests/phpunit/tests/user/retrievePassword.php

@@ -74,4 +74,17 @@ public function test_retrieve_password_should_fetch_user_by_login_if_not_found_b
 		$this->assertTrue( retrieve_password( '[email protected]' ), 'Fetching user by login failed.' );
 		$this->assertTrue( retrieve_password( '[email protected]' ), 'Fetching user by email failed.' );
 	}
+
+	/**
+	 * Tests that PHP 8.1 "passing null to non-nullable" deprecation notice
+	 * is not thrown when the `$user_login` parameter is empty.
+	 *
+	 * The notice that we should not see:
+	 * `Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated`.
+	 *
+	 * @ticket 62298
+	 */
+	public function test_retrieve_password_does_not_throw_deprecation_notice_with_default_parameters() {
+		$this->assertWPError( retrieve_password() );
+	}
 }