Preserve nil from JRuby getpass at EOF

[OskarEichler]

Summary

Use nil-safe chomp in both JRuby getpass implementations so EOF preserves MRI's nil contract instead of raising NoMethodError.

Reproduction

Focused external models return nil from gets; current code calls nil.chomp, while the candidate returns nil after restoring terminal output.

Verification

• external EOF models for both JRuby paths
• release/cumulative candidate: 36 tests / 143 assertions
• current upstream: 35 tests / 144 assertions
• syntax, package builds and Rails 8.1.3.1 loading

Compatibility

This restores MRI-compatible EOF behavior. JRuby was modeled but is unavailable locally; current upstream CI is green on JRuby. Prepared with AI-assisted source review; no repository tests were changed.