Fix logic if option param is not specified to null

src/AbstractPostSerializable.php

@@ -27,7 +27,7 @@ abstract class AbstractPostSerializable implements PostSerializableInterface {
         $vars = [];
 
         foreach ((new ReflectionClass(static::class))->getConstructor()->getParameters() as $param)
-            $vars[$param->getName()] = !$param->isOptional() || !$param->allowsNull();
+            $vars[$param->getName()] = !($param->isOptional() || $param->allowsNull());
 
         return $vars;
     }