fix null value in session

The null value may be 0 in the session.
Supports the case of 0

src/Rindow/Web/Session/Container.php

@@ -38,11 +38,12 @@ protected function onAccess()
             return;
         $this->connected = true;
         $old = $this->session->get($this->name);
-        if($old!==null) {
+        if($old) {
             $this->dict = $old;
         } else {
             $this->dict = new Dict();
         }
+
         $this->session->set($this->name,$this->dict);
     }
 
@@ -201,4 +202,4 @@ public function drop()
         $this->session->remove($this->name);
         $this->name = null;
     }
-}
+}