Add Null value support for RequiredUnless Validation

[BadJacky]

Description

In the spatie/laravel-data package, the RequireUnless validation rule does not adequately support null values.

This PR aims to address and rectify this issue by extending the rule's functionality to incorporate null value handling.

Changes

The RequireUnless class in the package has been updated as follows:

- public function __construct(
-     string|FieldReference                           $field,
-     array|string|BackedEnum|RouteParameterReference ...$values
- ) {
+ public function __construct(
+     string|FieldReference                                $field,
+     null|array|string|BackedEnum|RouteParameterReference ...$values
+ ) {
      $this->field = $this->parseFieldReference($field);
      $this->values = Arr::flatten($values);
  }

My PHP Code

<?php

namespace App\Data;

use App\Models\User;
use Illuminate\Validation\Rule;
use Spatie\LaravelData\Attributes\Validation\Email;
use Spatie\LaravelData\Attributes\Validation\Exists;
use Spatie\LaravelData\Attributes\Validation\Max;
use Spatie\LaravelData\Attributes\Validation\Min;
use Spatie\LaravelData\Attributes\Validation\Nullable;
use Spatie\LaravelData\Attributes\Validation\RequiredUnless;
use Spatie\LaravelData\Attributes\Validation\StringType;
use Spatie\LaravelData\Data;
use Spatie\LaravelData\Support\Validation\ValidationContext;
use Symfony\Contracts\Service\Attribute\Required;

class LoginFormData extends Data
{
    public function __construct(
        #[
            Nullable,
            RequiredUnless('email', null),
            StringType,
            Min(5),
            Max(20),
            Exists(User::class, 'username'),
        ]
        public readonly string $username,
        #[
            Nullable,
            RequiredUnless('username', null),
            StringType,
            Min(5),
            Max(20),
            Email(Email::RfcValidation),
            Exists(User::class, 'email')
        ]
        public readonly string $email,
        #[
            Required,
            StringType,
            Min(5),
            Max(20)
        ]
        public readonly string $password,
    ) {
    }

    /**
     * @return array<string, list<Rule|string>>
     */
    public static function rules(ValidationContext $context): array
    {
        return [
            'username' => ['required', 'string'],
            'password' => ['required', 'string'],
            'email'    => ['required', 'string', 'email', Rule::exists(User::class, 'email')],
        ];
    }
}

The Error Images

The screenshot after improved the RequireUnless

Impact

By extending support for null types in the RequireUnless validation rule, we gain finer control and flexibility while validating data. It also potentially resolves unforeseen issues or bugs related to the null value handling in future.

Related Documentation

require_unless in Laravel Framework

I appreciate your consideration of this PR and look forward to any feedback.

[rubenvanassche]

Thanks!