Adds the Rubocop ReturnNil cop

What does this MR do?

Adds the ReturnNil rubocop style check.

This is the EE companion to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25034/

If, after this is merged and rebased into your branch, you're seeing errors, you can fix them with

bundle exec rubocop . --only "Style/ReturnNil"  --auto-correct

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57454

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer