check-cherry-picks.sh maintainer script: fix handling of cherry-pick-less branches

maintainers/scripts/check-cherry-picks.sh

···
 problem=0
 
 while read 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