This change updates the output format of `git cl split` when
`description_file` has no footers.

Bug: 1215852
Change-Id: I69764885337ec31134f2b5e2d861930e0bc8cd2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2936161Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Fangzhen Song <songfangzhen@bytedance.com>