ci/check-cherry-picks: remove unused condition

The command substitution style we recently switched to strips trailing
newlines, so we don't need to check for empty lines anymore.

(cherry picked from commit 4c345b2fb4)
(cherry picked from commit 6388b8f4b5)

ci/check-cherry-picks.sh

@@ -20,9 +20,6 @@ remote="$(git remote -v | grep -i 'NixOS/nixpkgs' | head -n1 | cut -f1 || true)"
 commits="$(git rev-list --reverse "$1..$2")"
 
 while read -r new_commit_sha ; do
-  if [ -z "$new_commit_sha" ] ; then
-    continue  # skip empty lines
-  fi
   if [ "$GITHUB_ACTIONS" = 'true' ] ; then
     echo "::group::Commit $new_commit_sha"
   else