Test derivative case style for BeNil cop

[corsonknowles]

I was worried all the case statements with missing branch coverage would be like this, but some other ones at least are actually reachable code.

Solves for:

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• Set VersionChanged: "<<next>>" in config/default.yml.

[corsonknowles]

We should probably write up an Issue on the various causes of missing branch coverage, as well as the edge cases that we've solved for so far by filling out branch coverage.

As a reminder, having full branch coverage allows us to require full branch coverage, which could be quite useful as a standard moving forward to mitigate the introduction of unintended behaviors.