[PHP 8.4] Fix implicitly nullable type declarations

@fgm Fixed implicitly nullable type declarations in the code , Please review.

@sourabhsisodia_ can you also update the .gitlab-ci.yml to run the checks on PHP 8.4 ?

Also note that this is not just a base module issue, but applies to all the modules in the project. It's just associated to the base module because on d.o. one can only select a single component.

@fgm updated the MR according to recommended changes and also checked other submodules for nullable type declarations.

Thanks. As you can see, this MR fails https://git.drupalcode.org/issue/mongodb-3458044/-/pipelines/213633/test... so you now need to fix it.

@fgm can you please look into the phpunit error , I have tried but not able to solve.