Fix: Add explicit nullable typehints to resolve PHP deprecation warnings

[abavyka]

Description:

This Pull Request addresses deprecation warnings related to implicitly marked nullable parameters when running the package with newer PHP versions (specifically PHP 8.1+ up to 8.4, where this deprecation is fully enforced).

Problem:
PHP 8.1 introduced a deprecation notice (E_DEPRECATED) for parameters that are implicitly marked as nullable (e.g., function example($param = null) without a ? typehint). While still functional, this syntax will lead to errors in future PHP versions (e.g., PHP 9.0). This PR fixes warnings like:
Implicitly marking parameter {paramName} as nullable is deprecated, the explicit nullable type must be used instead

Solution:
The changes introduce explicit nullable typehints (?Type) for all parameters that can accept null as a value. This aligns the codebase with modern PHP best practices and eliminates the deprecation warnings.

[ZhukV]

Thank you for contributing! The version with your changes is v3.2.0.

Note: In this version, the minimum required PHP version is set to 7.4.