Laravel 5.4 sometimes|required validation not raising on "null" input

I'm having a problem validating inputs that are only going to be present sometimes in the Request.

// Controller
public function update(Request $request, User $user)
{    
    $updateResult = $user->updateUser($request);
    return dd($updateResult);
}

// User Model

protected $validation = [
    'rules' => [
        'email' => [
            'sometimes',
            'email',
            'required',
        ],
        'password' => [
            'sometimes',
            'min:6',
            'required',
        ],
        'first_name' => [
            'sometimes',
            'required',
        ],
        'last_name' => [
            'sometimes',
            'required',
        ],
    ],
    'messages' => [
        'email.required' => 'An email is required.',
        'email.email' => 'The email must be valid.',
        'password.required' => 'A password is required.',
        'password.min' => 'Your password must be at least six (6) characters long.',
        'first_name.required' => 'Your first name is required.',
        'last_name.required' => 'Your last name is required.',
    ],
];

public function updateUser(Request $request)
{
    $validation = Validator::make($request->all(), [
        $this->validation['rules'],
        $this->validation['messages'],
    ]);

    if ($validation->fails())
    {
        return $validation;
    }

    else
    {
        return "OK";
    }
}

So in some update pages $request->all() is only going to have a subset of these fields. However, even a field is present, but the value is null, the required doesn't trigger.

[
    'first_name' => null,
    'last_name' => 'Davidson',
    'job_title' => 'Tech Support',
]

The above request array will return "OK"... If I remove sometimes from the fields, then when a partial input request is sent, it fails saying the fields are required.

I am clearing missing something here, but from reading the docs I thought I'd configured this correctly:

In some situations, you may wish to run validation checks against a field only if that field is present in the input array. To quickly accomplish this, add the sometimes rule to your rule list:

$v = Validator::make($data, [ 'email' => 'sometimes|required|email', ]);

via Trent