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.

(cherry picked from commit ebf155ecc8)

.gitlab-ci.yml

@@ -731,7 +731,9 @@ changelog:
     - sed -i 's/^Draft:\s*//' commitmsg
     - echo -e "$CI_MERGE_REQUEST_DESCRIPTION" >> commitmsg
     - git commit --allow-empty -F commitmsg
-    - ./contrib/gitchangelog/gitchangelog.py HEAD^..HEAD >> $(ls doc/changelog/changelog-9.* | sort --version-sort | tail -n 1)
+    - export CHANGELOG=$(ls doc/changelog/changelog-9.* | sort --version-sort | tail -n 1)
+    - printf "\n" >> $CHANGELOG
+    - ./contrib/gitchangelog/gitchangelog.py HEAD^..HEAD >> $CHANGELOG
   after_script:
     - git diff
   needs: