[12.x] Improve docblocks for nullable parameters

[amirhshokri]

This PR updates docblocks for parameters with a default value of null that are currently documented as non-nullable.

• Follows the existing convention used elsewhere in the Laravel Framework source code.
• Applies the changes consistently across the codebase.

Due to the large volume of changes, these updates will be covered across 3 separate PRs (10 files each) for easier review.