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