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