fix(upsert): handle missing version.message in only-changed mode

[geopanther]

Summary

Guard against AttributeError when existing_page.version.message is absent on older self-hosted Confluence servers that don't populate version.message on page creation.

Adapted from iamjackg/md2cf#135 by @briandcho.

Changes

• Use hasattr() check before accessing version.message for both pages and attachments
• Treat missing message as 'needs update' (safe default)
• Add test for the no-message scenario

Tests

• New test: test_page_needs_updating_when_no_message
• All 17 upsert tests pass

[geopanther]

Maintainer Review

Verdict: ✅ Approve

Real-world bug fix. Older self-hosted Confluence (and potentially Cloud edge cases) may not populate version.message on initial page creation. The hasattr() guard is correct — when no message exists, there's no hash to compare, so treating it as 'needs update' is the safe default.

Good: Extended the same fix to attachment hash matching (line 192 in original), which the upstream PR didn't cover. This prevents the same crash when using --only-changed with attachments on these servers.

Suggestion for future: Consider wrapping this pattern in a small helper like get_version_hash(obj) to avoid repeating the hasattr + regex logic, but not blocking on that.

No concerns. Ship it.