Added NullTransformer

Author: andrey-helldar

config/feeds.php

@@ -111,5 +111,6 @@
         Transformers\BoolTransformer::class,
         Transformers\DateTimeTransformer::class,
         Transformers\EnumTransformer::class,
+        // Transformers\NullTransformer::class,
     ],
 ];

src/Transformers/NullTransformer.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DragonCode\LaravelFeed\Transformers;
+
+use DragonCode\LaravelFeed\Contracts\Transformer;
+
+class NullTransformer implements Transformer
+{
+    public function allow(mixed $value): bool
+    {
+        return $value === null;
+    }
+
+    public function transform(mixed $value): string
+    {
+        return 'null';
+    }
+}