Why {} is a not valid JSON and how to validate empty JSON?

I want to validate if the field I'm sending to the request is a valid JSON in Laravel (v9.x).

In the next example I just send { "test" : {} } as request content - dd($request):

^ Illuminate\Http\Request {#42 ▼
  +attributes: Symfony\Component\HttpFoundation\ParameterBag {#44 ▶}
  +request: Symfony\Component\HttpFoundation\ParameterBag {#34 ▶}
  +query: Symfony\Component\HttpFoundation\InputBag {#50 ▶}
  +server: Symfony\Component\HttpFoundation\ServerBag {#46 ▶}
  +files: Symfony\Component\HttpFoundation\FileBag {#47 ▶}
  +cookies: Symfony\Component\HttpFoundation\InputBag {#45 ▶}
  +headers: Symfony\Component\HttpFoundation\HeaderBag {#48 ▶}
  #content: "{"test":{}}"
  #languages: null
  #charsets: null
  #encodings: null
  #acceptableContentTypes: null
  #pathInfo: "/test"
  #requestUri: "/test"
  #baseUrl: ""
  #basePath: null
  #method: "POST"
  #format: null
  #session: Illuminate\Session\Store {#330 ▶}
  #locale: null
  #defaultLocale: "en"
  -preferredFormat: null
  -isHostValid: true
  -isForwardedValid: true
  #json: Symfony\Component\HttpFoundation\ParameterBag {#34 ▶}
  #convertedFiles: []
  #userResolver: Closure($guard = null) {#291 ▶}
  #routeResolver: Closure() {#300 ▶}
  basePath: ""
  format: "html"
}

But dd($request->get('test')); outputs:

^ []

Validation I'm doing:

$validator = Validator::make($request->all(), [
    'test' => ['present', 'json']
]);

Validation error I get doing dd($validator->errors()):

^ Illuminate\Support\MessageBag {#361 ▼
  #messages: array:1 [▼
    "test" => array:1 [▼
      0 => "The test must be a valid JSON string."
    ]
  ]
  #format: ":message"
}

Shouldn't be a valid JSON? Shouldn't it have been succeeded the validation?