`ComplianceManagement::PendingStatusCheckWorker` enqueued with `nil` as `diff_head_sha`

Summary

ComplianceManagement::PendingStatusCheckWorker has failed repeatedly as it got enqueued for a nil diff_head_sha, resulting in Validation failed: Sha can't be blank raised from within the worker. All these failures originate from POST /api/:version/projects/:id/merge_requests.

This is potentially triggered by the remove_source_branch param of the Update MR API endpoint.

Steps to reproduce

n/a

Example Project

n/a

What is the current bug behavior?

ComplianceManagement::PendingStatusCheckWorker enqueued for a nil diff_head_sha.

What is the expected correct behavior?

TBD

Relevant logs and/or screenshots

• https://new-sentry.gitlab.net/organizations/gitlab/issues/886926/?query=is%3Aunresolved+%22ComplianceManagement%3A%3APendingStatusCheckWorker%22&referrer=issue-stream&statsPeriod=14d&stream_index=0
• KQL: json.class:"ComplianceManagement::PendingStatusCheckWorker" AND json.job_status:"fail"

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes