Prevent empty session creation (#2821)

* Prevent empty sessions from being created. 6a5ad31
* Update class.session.php with `!` instead of ` == ''` 1ee976b

Closes #2820

symphony/lib/core/class.session.php

@@ -183,7 +183,7 @@ public static function write($id, $data)
         // in the database. This prevents empty Sessions from being created, but
         // allows them to be nulled.
         $session_data = Session::read($id);
-        if (is_null($session_data)) {
+        if (!$session_data) {
             $empty = true;
             $unserialized_data = Session::unserialize($data);