Fix array_map() warning.

Basically the same as voceconnect#22, but placed in a better location.

wp-multisite-sso.php

@@ -141,6 +141,7 @@ public static function handle_login( $username, $user ) {
 			$user_agent = wp_unslash( $_SERVER['HTTP_USER_AGENT'] );
 		}
 
+		$sso_objects = array();
 		foreach( array_keys( $network_sites ) as $blog_id ) {
 			// build the sso objects to send
 			$sso_objects[$blog_id] = array(