refactor: explicit nullable parameters

[joostdebruijn]

1️⃣ Why should it be added? What are the benefits of this change?

This PR makes Laravel-Excel compatible with PHP 8.4 without throwing deprecation warnings while maintaining backwards compatibility. There is a similar PR (#4248), but that one is also incorporating some breaking changes.

2️⃣ Does it contain multiple, unrelated changes? Please separate the PRs out.

No.

3️⃣ Does it include tests, if possible?

Tests are already running for 8.4.

4️⃣ Any drawbacks? Possible breaking changes?

No.

5️⃣ Mark the following tasks as done:

• Checked the codebase to ensure that your feature doesn't already exist.
• Take note of the contributing guidelines.
• Checked the pull requests to ensure that another person hasn't already submitted a fix.
• Added tests to ensure against regression.

6️⃣ Thanks for contributing! 🙌

[patrickbrouwers]

Thanks!