fix a few more session-related warnings

backend.php

@@ -46,7 +46,7 @@
 		UserHelper::authenticate( "admin", null);
 	}
 
-	if ($_SESSION["uid"]) {
+	if (!empty($_SESSION["uid"])) {
 		if (!validate_session()) {
 			header("Content-Type: text/json");
 			print error_json(6);

classes/handler/protected.php

@@ -2,6 +2,6 @@
 class Handler_Protected extends Handler {
 
 	function before($method) {
-		return parent::before($method) && $_SESSION['uid'];
+		return parent::before($method) && !empty($_SESSION['uid']);
 	}
 }