Fix the nullable parameter declarations for compatibility with PHP 8.4

[martio]

Hi! 😃

I have prepared a small fix that resolves the following issue:

DEPRECATED  Yiisoft\Translator\TranslatorInterface::translate(): Implicitly marking parameter $locale as nullable is deprecated, the explicit nullable type must be used instead in vendor/yiisoft/translator/src/TranslatorInterface.php on line 46.

Yiisoft\Translator\TranslatorInterface

   public function translate(
       string|Stringable $id,
       array $parameters = [],
       string $category = null,
       string $locale = null
   ): string;

What steps will reproduce the problem?

Attempt to use the Yiisoft\Translator\TranslatorInterface::translate method in a PHP 8.4 environment without explicitly declaring nullable types for parameters like $locale.

What is the expected result?

The translate method should function without deprecation warnings, and the application should remain compatible with PHP 8.4.

What do you get instead?

A deprecation warning is triggered:
DEPRECATED: Yiisoft\Translator\TranslatorInterface::translate(): Implicitly marking parameter $locale as nullable is deprecated, the explicit nullable type must be used instead in vendor/yiisoft/translator/src/TranslatorInterface.php on line 46.`

Additional info

Q	A
Version	2.1.0
PHP version	8.4.1
Operating system	Unix

[vjik]

Done by #148