Resource fails when MR with deleted source branch exists

[detached]

When in Gitlab a merge request is open but its source branch was deleted, the Gitlab API returns the following json:

{
        "id": 105,
        "iid": 47,
        "project_id": 21,
        "title": "Feature/xxxxx",
        "description": "xxxxx",
        "state": "opened",
        "created_at": "2017-05-03T13:55:50.786Z",
        "updated_at": "2017-05-08T09:36:02.817Z",
        "target_branch": "development",
        "source_branch": "feature/xxxx",
        "upvotes": 0,
        "downvotes": 0,
        "author": {
            "name": "xxxxxx",
            "username": "xxxxx",
            "id": 8,
            "state": "active",
            "avatar_url": "xxxxx",
            "web_url": "xxxxx"
        },
        "assignee": {
            "name": "xxxxxx",
            "username": "xxxxx",
            "id": 8,
            "state": "active",
            "avatar_url": "xxxxx",
            "web_url": "xxxxx"
        },
        "source_project_id": 21,
        "target_project_id": 21,
        "labels": [

        ],
        "work_in_progress": false,
        "milestone": null,
        "merge_when_build_succeeds": false,
        "merge_status": "unchecked",
        "sha": null,
        "merge_commit_sha": null,
        "subscribed": false,
        "user_notes_count": 2,
        "should_remove_source_branch": null,
        "force_remove_source_branch": false,
        "web_url": "http://gitlab.company.de/proj/path/merge_requests/47"
    }

Because the sha value is null the resource is failing in the check step and no merge request can be build.

resource script '/opt/resource/check []' failed: exit status 128

stderr:
Already up-to-date.
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100  3440  100  3440    0     0  19355      0 --:--:-- --:--:-- --:--:-- 19545
fatal: ambiguous argument 'null': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

The problematic line is https://github.com/swisscom/gitlab-merge-request-resource/blob/master/scripts/check#L72

Maybe it would be good to check for this case and just skip this merge request with a warning message.

PS:
Thank you for the plugin, it works great! 👍

[cloudlena]

Thanks, @detached! I created #10. Please let me know if you think that would solve your problem.

[cloudlena]

Should be fixed by #11. If so, please close.

[detached]

Thank you!