fix(res.links): skip empty-array rel values to prevent trailing comma in Link header

[JSap0914]

Bug

When res.links() is called with an empty array [] for a rel value, the inner .map().join(', ') produces an empty string ''. The outer .join(', ') then includes that empty entry, yielding an invalid Link header:

res.links({ next: 'http://example.com/2', prev: [] })
// Before: Link: <http://example.com/2>; rel="next",   ← trailing comma, invalid
// After:  Link: <http://example.com/2>; rel="next"

A second call appending only empty-array rels also produced a trailing comma:

res.links({ next: 'http://example.com/2' })
res.links({ prev: [] })
// Before: Link: <http://example.com/2>; rel="next",
// After:  Link: <http://example.com/2>; rel="next"

RFC 8288 requires the Link header to be a comma-separated list of link-values; a trailing comma or empty entry is invalid and may confuse clients or intermediaries.

Fix

Extract the new link entries into newLinks, filter out empty strings with .filter(Boolean), then only prepend ', ' when both the existing Link value and the new entries are non-empty.

Verification

npx mocha --require test/support/env --reporter spec --check-leaks test/res.links.js

Note: This fix was developed with AI-assist.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.