[5.8] Don't check for status code in assertJsonMissingValidationErrors()

[SjorsO]

#28595 fixes a problem where returning an empty response causes the assertJsonMissingValidationErrors assertion to fail because an empty response is not valid json.

This PR removes the status code 204 check. With this PR you can write tests using assertJsonMissingValidationErrors without explicitly returning a response. For example:

Controller:

public function post(Request $request)
{
    $data = $request->validate([
        // a bunch of validation rules
    ]);

    SomeModel::create($data);
}

Test:

function it_stores_the_model()
{
    $this->postJson('/model/store', [
            // a bunch of values
        ])
        ->assertJsonMissingValidationErrors()
        ->assertStatus(200);
}

Using a combination of these two assertions is useful because if the validation fails, the test will output exactly what went wrong. It also checks if your code explodes and causes a 500 error.

Often when writing api endpoints, you don't return a response. It is either not necessary, or because the consumer should just do another index call. However, currently the test above fails because we are not explicitly returning a 204 status code.