fix(zip): suppress sabre/dav response only if stream was actually sent

[solracsf]

Summary

afterDownload currently returns false (suppressing sabre/dav's own response logic) for any GET request targeting a Directory node, regardless of whether handleDownload actually streamed an archive.

This means a GET on a folder without an Accept: zip or Accept: tar header causes handleDownload to correctly bail out with null, but afterDownload then silently swallows sabre/dav's response, resulting in an empty reply to the client instead of the expected.

The fix: suppress sabre/dav response only if stream was actually sent.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI