Preserve nil values in array-valued form fields

[OskarEichler]

Summary

Return the encoded key after processing each array element so a nil value produces a key-only field, matching the existing scalar-nil behavior.

Reproduction

URI.encode_www_form(foo: [nil, 'a', nil]) returns &foo=a& before and foo&foo=a&foo after. The map block previously returned nil whenever its unless branch was skipped.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 93 tests / 3,039 assertions / zero failures or errors, both baseline and this isolated patch.
• 148 checks compare array elements with independently encoded scalar fields across nil/empty/string/numeric/false values and empty arrays.
• Syntax and git diff --check pass. Supplemental Lint adds no findings against the 31-finding baseline.
• No test/spec files added or modified, per this contribution's explicit no-new-tests constraint. Focused reproductions ran externally.

Compatibility and limitations

Intentional correction: nil array elements retain their keys. Existing scalar nil and empty-array behavior is unchanged. No public API removal, dependency change or Ruby minimum change.

Based on master 696df43b0be4c05d3a0dcf7db582126c915d860d. Other Ruby/OS runtimes were not executed locally. No production traffic or live mail/FTP services were used. Existing related PRs/issues were checked; this is a focused contribution, not exhaustive behavioral coverage.