[9.20] chg: ci: Add newline for changelog CI job

In case the changelog file doesn't have an empty line at the end of the
file, the job may fail with the following error:

WARNING: Bullet list ends without a blank line; unexpected unindent.

This typically happens in MRs targeting the -S edition, as those
changelogs usually don't have an empty newline. This change ensures the
changelog job can pass and verify the title/desc contents even in those
cases.

Backport of MR !10628

Merge branch 'backport-nicki/ci-changelog-add-missing-newline-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10632