Fix bug when empty case nodes are present

[tiagocasanovapt]

When defined inside a function, an empty when block from a case switch caused line 79 at lib/rubocop/cop/style/redundant_return.rb to fail due to a nil node.

I didn't add an IF in line 96 (check_case_node) to match similar cases because it failed to pass the tests, namely on line length.

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Before submitting the PR make sure the following are checked:

• Documentation updated with rake generate_cops_documentation
• Wrote good commit messages.
• [ ] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
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• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

Fix bug when empty case nodes are present

[bbatsov]

The fix looks good, but the commit message doesn't match our guidelines.

[tiagocasanovapt]

Sorry, I assumed "Bullet points are okay, too", which i'm more used to. I've edited the commit message and the PR title.

[bbatsov]

One more thing - you forgot to add yourself to the bottom of the changelog. You'll also need to rebase your PR on top of the current master.

[tiagocasanovapt]

Done & done. Please review. 👍