Fix to strict param type

fixes #41

Author: fetzi

CHANGELOG.md

@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [4.0.3] - 2020-11-30
+### Fixed
+- To strict param type in `transform` method
+
 ## [4.0.2] - 2020-10-15
 ### Fixed
 - Another issue where CallbackBinding wasn't handled.

src/JsonDecoder.php

@@ -137,7 +137,7 @@ public function shouldAutoCase(): bool
      *
      * @throws JsonValueException if the json data is not valid
      */
-    protected function transform(Transformer $transformer, array $jsonArrayData, $instance)
+    protected function transform(Transformer $transformer, ?array $jsonArrayData, $instance)
     {
         if (empty($jsonArrayData)) {
             return null;