Fix nullable deprecated warnings

[crodriguezbrito]

Proposed changes

This PR fix the Implicitly marking parameter $callback as nullable is deprecated, the explicit nullable type must be used instead notice on PHP8.4

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments