[Bugfix] Allow null as data for to-one relationship (#197)

This allows you to remove a to-one relationship by sending `null` in a PATCH request.

Author: JaZo

Diff for: src/Http/Requests/ResourceRequest.php

@@ -204,7 +204,7 @@ public function document(): array
     {
         $document = $this->json()->all();
 
-        if (!is_array($document) || !isset($document['data']) || !is_array($document['data'])) {
+        if (!is_array($document) || !array_key_exists('data', $document) || !(is_array($document['data']) || is_null($document['data']))) {
             throw new LogicException('Expecting JSON API specification compliance to have been run.');
         }