fix: TypeError on PHP8

TypeError: ReflectionProperty::getValue(): Argument #1 ($object) must be of type ?object, string given

Author: kenjis

src/ReflectionHelper.php

@@ -80,6 +80,10 @@ public static function getPrivateProperty($obj, string $property)
     {
         $refProperty = self::getAccessibleRefProperty($obj, $property);
 
-        return $refProperty->getValue($obj);
+        if (is_object($obj)) {
+            return $refProperty->getValue($obj);
+        }
+
+        return $refProperty->getValue();
     }
 }