Users: wp_signon() expects an array as the $credentials argument, not a string.

If an empty string was passed, redeclare it as an empty array to avoid a warning and a fatal error in PHP 7.1.0 Alpha 1.

Props simonvik.
Fixes #37071.

git-svn-id: https://develop.svn.wordpress.org/trunk@37697 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-login.php

@@ -790,7 +790,7 @@ default:
 
 	$reauth = empty($_REQUEST['reauth']) ? false : true;
 
-	$user = wp_signon( '', $secure_cookie );
+	$user = wp_signon( array(), $secure_cookie );
 
 	if ( empty( $_COOKIE[ LOGGED_IN_COOKIE ] ) ) {
 		if ( headers_sent() ) {