[9.x]: Add whenNull to ConditionallyLoadsAtrribute trait (#43600)

* [9.x]: Add whenNull to ConditionallyLoadsAtrribute

* Update ConditionallyLoadsAttributes.php

Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Http/Resources/ConditionallyLoadsAttributes.php

@@ -157,6 +157,20 @@ protected function attributes($attributes)
         );
     }
 
+    /**
+     * Retrieve a model attribute if it is null.
+     *
+     * @param  mixed  $value
+     * @param  mixed  $default
+     * @return \Illuminate\Http\Resources\MissingValue|mixed
+     */
+    protected function whenNull($value, $default = null)
+    {
+        $arguments = func_num_args() == 1 ? [$value] : [$value, $default];
+
+        return $this->when(is_null($value), ...$arguments);
+    }
+
     /**
      * Retrieve a model attribute if it is not null.
      *