Style Change Proposal: Add Rubocop ReturnNil cop

Description of the proposal

Spawned from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24902#note_139520959

I had the following:

return nil unless tracing_url_enabled?

@grzesiek reviewed with:

nil is redundant here.

Which made me wonder why we're not enforcing this with a cop instead of relying on maintainers to tell us.

Bad Style ❌

return nil unless condition

Good Style ✅

return unless condition

• Mention the proposal in the next backend weekly call and the #backend channel to encourage contribution
• Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are 👍
• Once approved, mention it again in the next backend weekly call and the #backend channel

Maintainer Voting

Maintainer	Nays to the left	Ayes to the right
@ayufan		👍
@dbalexandre		👍
@DouweM		👍
@dzaporozhets		👍
@godfat
@grzesiek		👍
@nick.thomas		👍
@rspeicher		👍
@rymai		👍
@smcgivern		👍
@stanhu		👍

/cc @gitlab-org/maintainers/rails-backend

Merge Request: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25034