How to specify a nullable value on create function in modal in Laravel?

In a model, how are we able to insert a nullable validated field with the create function? For example, in the RegisterController: the basic validator + the create function. If I didn't send the google_id or facebook_id, the create function returns an undefined index (obviously) on the fields. What's the proper way to insert them?

protected function validator(array $data)
{
    return Validator::make($data, [
        'name' => ['required', 'string', 'max:255'],
        'email' => ['nullable', 'email', 'unique:users'],
        'google_id' => ['nullable', 'string', 'unique:users'],
        'facebook_id' => ['nullable', 'string', 'unique:users'],
        'twitter_id' => ['nullable', 'string', 'unique:users'],
        'phone' => ['required', 'string', 'max:10', 'min:10', 'unique:users', "regex:/(^[1-9][0-9]*$)/"],
        'country_code' => ['required', 'string', 'max:255', 'exists:'.config('countries.table_name').',calling_code'],
        'password' => ['required', 'string', 'min:8'],
    ]);
}

/**
 * Create a new user instance after a valid registration.
 *
 * @param  array  $data
 * @return \App\User
 */
protected function create(array $data)
{
    return User::create([
        'name' => $data['name'],
        'email' => $data['email'],
        'phone' => $data['country_code'].$data['phone'],
        'google_id' => $data['google_id'] ? $data['google_id'] : null,
        'facebook_id' => $data['facebook_id'] ? $data['facebook_id'] : null,
        'twitter_id' => $data['twitter_id'] ? $data['twitter_id'] : null,
        'password' => Hash::make($data['password']),
    ]);
}

I know it's a fundamental question, and I do know how to ignore them, but I need to know what the correct format is.