PHP 8.4 Deprecation warnings

## Feature or problem being solved Adding explicit nullable to parameters since implicit nullable parameters are deprecated in PHP 8.4 Affected code: InvalidUserProvider::L09 MissingUserProvider::L09 ### Further details Adding the explicit nullable parameter, gets rid of unnecessary deprecation warnings for php 8.4. ## Links / references Implicitly nullable values are deprecated in php 8.4. See: https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated