message: Fix exception when there are no commits to merge
diff: @@ -98,7 +98,9 @@ def open_pr(repo, all_commits, short_main_sha, new_branch_name, source_branch, t
  # This will not include any commits that exist on the release branch
  # that aren't on main.
  def get_commit_difference(repo, source_branch, target_branch):
  - commits = run_git('log', '--pretty=format:%H', ORIGIN + '/' + target_branch + '..' + ORIGIN + '/' + source_branch).strip().split('\n')
  + # Passing split nothing means that the empty string splits to nothing: compare `''.split() == []`
  + # to `''.split('\n') == ['']`.
  + commits = run_git('log', '--pretty=format:%H', ORIGIN + '/' + target_branch + '..' + ORIGIN + '/' + source_branch).strip().split()
  # Convert to full-fledged commit objects
  commits = [repo.get_commit(c) for c in commits]
  