Change to explicit nullable type to avoid rfc/deprecate-implicitly-nullable-types warning

[nukisashineko]

Summary

PHP 8.4 now displays a Deprecated warning for defining an implicit nullable type by assignment.
link: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types
I would like this warning to be fixed, as it makes it difficult to see the results of tests when this warning is displayed continuously.

How to reproduce

The warning can be confirmed by running test.

Purpose

I am using this project to write tests.
php 8.4 has been released.
After updating to php 8.4, we found that a warning is displayed and interferes with the display of test results.

Tasks.

• Cover the strategy matrix of github actions up to php 8.4.
• Change implicit nullable type definitions to explicit nullable type definitions.

Miscellaneous

PullRequest has been created.
I would appreciate your confirmation.

PR

• Fix warning for rfc/deprecate-implicitly-nullable-types and Add php version 8.2 ~ 8.4 to strategy matrix  #62