Conflict_resolution draft_commit_conflicts doesn't open draft PR on empty commit due to conflicts

[korthout]

Describe the bug
The experimental conflict_resolution input value draft_commit_conflicts should open a draft PR if there are conflicts. However, if the cherry-picked commit is empty then no draft PR is opened.

To Reproduce
Steps to reproduce the behavior:

1. Create and merge a pull request with some changes
2. Trigger the backport workflow to a branch
3. Merge the backport PR and delete its branch
4. Trigger the backport workflow to the same branch again
5. Notice an error like:

Backport failed for main, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin main
git worktree add -d .worktree/backport-28924-to-main origin/main
cd .worktree/backport-28924-to-main
git switch --create backport-28924-to-main
git cherry-pick -x 5b7caeed208862b2c601a5905ac5f80f0c1fb67c

Expected behavior

An empty cherry-picked commit indicates that the target branch already contains the changes of the relevant commit.

We can:

• create a draft PR where the user has to resolve the conflict themselves
  • we should do this on draft_commit_conflicts
• allow skipping empty commits directly with a new input empty_commits: skip|fail similar to merge_commits

Workflow run logs
If available, add logs from the workflow run where you noticed this bug to help explain your problem.

Workflow run logs

2025-03-03T23:39:01.3451789Z Retrieve pull request data for #28924
2025-03-03T23:39:01.7536243Z Check whether pull request 28924 is merged
2025-03-03T23:39:01.8797684Z Determining target branches...
2025-03-03T23:39:01.8798448Z Detected labels on PR: backport main,component/zeebe
2025-03-03T23:39:01.8800613Z label 'component/zeebe' doesn't match `label_pattern` '^backport ([^ ]+)$'
2025-03-03T23:39:01.8801595Z Found target branches in labels: main
2025-03-03T23:39:01.8802193Z Found target branches in `target_branches` input: 
2025-03-03T23:39:01.8803295Z Exclude pull request's headref from target branches: backport-28865-to-stable/8.7
2025-03-03T23:39:01.8804323Z Determined target branches: main
2025-03-03T23:39:01.8805149Z Fetching all the commits from the pull request: 2
2025-03-03T23:39:01.8805864Z git fetch --depth=2 origin refs/pull/28924/head
2025-03-03T23:39:02.5441897Z From https://github.com/camunda/camunda
2025-03-03T23:39:02.5442644Z  * branch              refs/pull/28924/head -> FETCH_HEAD
2025-03-03T23:39:02.5486813Z Retrieving the commits from pull request 28924
2025-03-03T23:39:02.8993460Z PR was merged using a merge commit
2025-03-03T23:39:02.8994267Z Found commits to backport: 5b7caeed208862b2c601a5905ac5f80f0c1fb67c
2025-03-03T23:39:02.9002268Z Checking the merged pull request for merge commits
2025-03-03T23:39:02.9003271Z git rev-list --merges 5b7caeed208862b2c601a5905ac5f80f0c1fb67c^..5b7caeed208862b2c601a5905ac5f80f0c1fb67c
2025-03-03T23:39:02.9041776Z Encountered 0 merge commits
2025-03-03T23:39:02.9043078Z Will cherry-pick the following commits: 5b7caeed208862b2c601a5905ac5f80f0c1fb67c
2025-03-03T23:39:02.9043917Z Will copy labels matching undefined. Found matching labels: 
2025-03-03T23:39:02.9044604Z Backporting to target branch 'main...'
2025-03-03T23:39:02.9045107Z git fetch --depth=1 origin main
2025-03-03T23:39:07.6986785Z From https://github.com/camunda/camunda
2025-03-03T23:39:07.6987552Z  * branch              main       -> FETCH_HEAD
2025-03-03T23:39:07.6989741Z  * [new branch]        main       -> origin/main
2025-03-03T23:39:07.7031529Z Start backport to backport-28924-to-main
2025-03-03T23:39:07.7032652Z git switch -c backport-28924-to-main origin/main
2025-03-03T23:39:08.5261444Z Switched to a new branch 'backport-28924-to-main'
2025-03-03T23:39:08.5312517Z git cherry-pick -x 5b7caeed208862b2c601a5905ac5f80f0c1fb67c
2025-03-03T23:39:09.3163533Z The previous cherry-pick is now empty, possibly due to conflict resolution.
2025-03-03T23:39:09.3165009Z If you wish to commit it anyway, use:
2025-03-03T23:39:09.3165418Z 
2025-03-03T23:39:09.3165620Z     git commit --allow-empty
2025-03-03T23:39:09.3165948Z 
2025-03-03T23:39:09.3166210Z Otherwise, please use 'git cherry-pick --skip'
2025-03-03T23:39:09.3207079Z git commit --all -m BACKPORT-CONFLICT
2025-03-03T23:39:09.4098126Z The previous cherry-pick is now empty, possibly due to conflict resolution.
2025-03-03T23:39:09.4098985Z If you wish to commit it anyway, use:
2025-03-03T23:39:09.4099963Z 
2025-03-03T23:39:09.4100253Z     git commit --allow-empty
2025-03-03T23:39:09.4100754Z 
2025-03-03T23:39:09.4103947Z Otherwise, please use 'git cherry-pick --skip'
2025-03-03T23:39:09.4117598Z git cherry-pick --abort
2025-03-03T23:39:09.4366056Z Backport failed for `main`, because it was unable to cherry-pick the commit(s).
2025-03-03T23:39:09.4366673Z 
2025-03-03T23:39:09.4366982Z Please cherry-pick the changes locally and resolve any conflicts.
2025-03-03T23:39:09.4367584Z ```bash
2025-03-03T23:39:09.4367902Z git fetch origin main
2025-03-03T23:39:09.4368434Z git worktree add -d .worktree/backport-28924-to-main origin/main
2025-03-03T23:39:09.4369071Z cd .worktree/backport-28924-to-main
2025-03-03T23:39:09.4369568Z git switch --create backport-28924-to-main
2025-03-03T23:39:09.4370617Z git cherry-pick -x 5b7caeed208862b2c601a5905ac5f80f0c1fb67c
2025-03-03T23:39:09.4371390Z ```
2025-03-03T23:39:09.4372036Z Create comment: Backport failed for `main`, because it was unable to cherry-pick the commit(s).
2025-03-03T23:39:09.4372672Z 
2025-03-03T23:39:09.4372998Z Please cherry-pick the changes locally and resolve any conflicts.
2025-03-03T23:39:09.4373605Z ```bash
2025-03-03T23:39:09.4373926Z git fetch origin main
2025-03-03T23:39:09.4374433Z git worktree add -d .worktree/backport-28924-to-main origin/main
2025-03-03T23:39:09.4375046Z cd .worktree/backport-28924-to-main
2025-03-03T23:39:09.4375529Z git switch --create backport-28924-to-main
2025-03-03T23:39:09.4376118Z git cherry-pick -x 5b7caeed208862b2c601a5905ac5f80f0c1fb67c
2025-03-03T23:39:09.4376669Z

Additional context

• [Backport stable/8.7] Adjust inconsistent OpenAPI attribute names camunda/camunda#28924 (comment)
• https://github.com/camunda/camunda/actions/runs/13642730658/job/38135838568