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

### 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
- https://github.com/Codeception/Specify/pull/62

Start by running the test suite on PHP 8.4 to reproduce the deprecation warning, then locate the implicit nullable type definitions mentioned in the task. Update the GitHub Actions strategy through PHP 8.4 and verify that the tests run without the warning.

Written by the indexing model from the issue text.