Fix errors for PHPStan level 3

Author: Art4

.github/workflows/phpstan.yml

@@ -49,6 +49,6 @@ jobs:
         run: composer install --no-progress --prefer-dist --optimize-autoloader
 
       - name: Run PHPStan
-        run: vendor/bin/phpstan.phar analyze src --level 2
+        run: vendor/bin/phpstan.phar analyze src --level 3
 
       - run: echo "🍏 This job's status is ${{ job.status }}."

src/Helper/AccessKey.php

@@ -33,7 +33,7 @@ final class AccessKey extends SplStack
      *
      * @param mixed $key
      *
-     * @return string
+     * @return AccessKey
      */
     public static function create($key)
     {

src/Serializer/ArraySerializer.php

@@ -47,7 +47,7 @@ public function __construct(array $params = [])
      *
      * @param \Art4\JsonApiClient\Accessable $data The data for serialization
      *
-     * @return array
+     * @return array|null
      */
     public function serialize(Accessable $data)
     {