Handle case when node.receiver is nil in Style/InvertibleUnlessCondition

[shioyama]

Style/InvertibleUnlessCondition changes in #12562 caused this cop to fail in the case where the node receiver is nil. See the failing test. I made a fix.

• 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.

[shioyama]

Two complexity metrics are failing. I fundamentally disagree with these so if you'd like to fix them, please edit the changes:

Offenses:

lib/rubocop/cop/style/invertible_unless_condition.rb:102:9: C: Metrics/CyclomaticComplexity: Cyclomatic complexity for preferred_send_condition is too high. [8/7]
        def preferred_send_condition(node) ...
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/rubocop/cop/style/invertible_unless_condition.rb:102:9: C: Metrics/PerceivedComplexity: Perceived complexity for preferred_send_condition is too high. [9/8]
        def preferred_send_condition(node) ...
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[shioyama]

To me a series of if/else is much clearer than a bunch of guard clauses.

[shioyama]

cc @sambostock

[sambostock]

🤦 I missed your ping @shioyama and ended up coincidentally opening #12711 to fix the same bug. I'll leave it open for comparison though, as the approach is slightly different, and it has a couple more tests.

[shioyama]

Thanks! I'll close this, no worries.