Fix Rails/StrongParametersExpect to allow safe navigation operator on optional params

[lucasmazza]

&. is semantically equivalent to || nil — both communicate that the developer expects the param may be absent. The cop already exempts || parent nodes via or_type?; this adds the same guard for csend_type? (safe navigation receiver).

Without this fix, params[:key]&.downcase is autocorrected to params.expect(:key).downcase, silently changing behavior: a missing param goes from returning nil to raising ActionController::ParameterMissing (400). The developer used &. precisely to handle the nil case — the autocorrect removes that intent.

The guard is scoped to parent.receiver == node so Model&.find(params[:id]) (where params[:id] is an argument, not the receiver of the safe navigation) is still correctly flagged.

---

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

[koic]

@lucasmazza This looks good to me. Can you squash your commits into one?

[lucasmazza]

@koic done

[koic]

Thanks!