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.
Built from https://develop.svn.wordpress.org/trunk@37697


git-svn-id: http://core.svn.wordpress.org/trunk@37663 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -27,6 +27,8 @@
  */
 function wp_signon( $credentials = array(), $secure_cookie = '' ) {
 	if ( empty($credentials) ) {
+		$credentials = array(); // Back-compat for plugins passing an empty string.
+
 		if ( ! empty($_POST['log']) )
 			$credentials['user_login'] = $_POST['log'];
 		if ( ! empty($_POST['pwd']) )