fix(upgrade): indent changelog, add emoji to heading, hide empty sections (#604)

## Summary

Follow-up to #594. Fixes changelog rendering issues spotted during first
real nightly upgrade:

- Indent all changelog lines by 2 spaces to visually nest under the
upgrade status header
- Add ✨ emoji to the "What's new" heading for consistency with category
headings
- Skip sections whose markdown is empty after whitespace trimming
- Add trailing newline so the output doesn't butt against the shell
prompt

Author: BYK