fix(api-response-facade): Add castToNull method declaration

- Introduce the castToNull() method to the ApiResponseFacade.
- This method provides a way to handle null casting in API responses.
- Enhances the functionality of the facade for better type handling.

src/Facades/ApiResponseFacade.php

@@ -30,6 +30,7 @@
  * @method static \Guanguans\LaravelApiResponse\ApiResponse castToBoolean()
  * @method static \Guanguans\LaravelApiResponse\ApiResponse castToFloat()
  * @method static \Guanguans\LaravelApiResponse\ApiResponse castToInteger()
+ * @method static \Guanguans\LaravelApiResponse\ApiResponse castToNull()
  * @method static \Guanguans\LaravelApiResponse\ApiResponse castToObject()
  * @method static \Guanguans\LaravelApiResponse\ApiResponse castToString()
  * @method static \Illuminate\Http\JsonResponse conflict(string $message = '')

src/Pipes/CallableDataPipe.php

@@ -46,6 +46,8 @@ public function handle(array $structure, \Closure $next): JsonResponse
      */
     private function dataFor($data)
     {
-        return \is_callable($data) && !(\is_string($data) && \function_exists($data)) ? $data() : $data;
+        return \is_callable($data) && !(\is_string($data) && \function_exists($data))
+            ? $data()
+            : $data;
     }
 }