fix: add default null value for json-api property on document class

Closes #5

Author: lindyhopchris

CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to this project will be documented in this file. This projec
 
 ## Unreleased
 
+### Fixed
+
+- [#5](https://github.com/laravel-json-api/encoder-neomerx/issues/5) Fix error with default value for `$jsonApi`
+  property on the `Document` class.
+
 ## [3.1.0] - 2023-11-08
 
 ### Changed

src/Document.php

@@ -40,7 +40,7 @@ abstract class Document implements JsonApiDocument
     /**
      * @var JsonApi|null
      */
-    private ?JsonApi $jsonApi;
+    private ?JsonApi $jsonApi = null;
 
     /**
      * @var Links|null