Fix Rails/FilePath cop error on join method with implicit receiver

[viralpraxis]

echo 'join(Rails.root, path)' | rubocop --only Rails/FilePath  --stdin bug.rb -d

n error occurred while Rails/FilePath cop was inspecting bug.rb:1:0.
undefined method `each_node' for nil
lib/rubocop/cop/rails/file_path.rb:54:in `rails_root_nodes?'
lib/rubocop/cop/rails/file_path.rb:60:in `rails_root_join_nodes?'
lib/rubocop/cop/rails/file_path.rb:108:in `check_for_rails_root_join_with_string_arguments'
lib/rubocop/cop/rails/file_path.rb:70:in `on_send'

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• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Test and implementation like this seem to be required for both EnforcedStyle: 'arguments' and EnforcedStyle: 'slashes'.

[viralpraxis]

Good point, fixed.

[koic]

Thanks!