Fix an error for Minitest/SkipEnsure when only ensure has a body

[Earlopain]

Found at https://github.com/Shopify/shopify_app/blob/1dda5cb5642e34338961cc65d8069455e568e162/test/shopify_app/controller_concerns/token_exchange_test.rb#L30-L35

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

Thanks!