Build status not displayed on the Pull Request list page when the new build API does not include a ref

Issue Summary

Build status is not displayed on the Pull Request list page when the new build API does not include a ref (from branch)
This is reproducible on Data Center: yes

Steps to Reproduce

1. Create a pull request
2. A build is fired associated with a commit in the PR
3. The notification back to BB Server uses the new build API

   /rest/api/1.0/projects/{projectKey}/repos/{repositorySlug}/commits/{commitId}/builds
   

   and the body does not include a ref (such as refs/heads/branch)

   {
       "key": "TEST-3",
       "state": "SUCCESSFUL",
       "url": "https://bamboo.url/browse/TEST-3",
       "buildNumber": "3",
       "description": "test without ref",
       "duration": 1500000,
       "name": "build without a ref",
       "parent": "TEST",
       "testResults": {
           "failed": 0,
           "skipped": 0,
           "successful": 1
       }
   }
   

Expected Results

The build is shown on the PR Overview, PR builds, and PR list pages

Actual Results

The build is shown on the PR Overview, PR builds but NOT on the PR list page

Workaround

One would need to add the ref on the body of the notification

{
    "key": "TEST-2",
    "state": "SUCCESSFUL",
    "url": "https://bamboo.url/browse/TEST-2",
    "buildNumber": "2",
    "description": "test without ref",
    "duration": 1500000,
    "name": "build with a ref",
    "parent": "TEST",
    "ref": "refs/heads/branch2",
    "testResults": {
        "failed": 0,
        "skipped": 0,
        "successful": 1
    }
}

Note: This affects only BB 7+versions. This behavior is not exhibited in BB 8+