Pass diff files when "Merge request events" trigger a webhook

version: GitLab Enterprise Edition 9.5.9-ee.

I use GitLab webhook to trigger Jenkins CI when there are "Merge request events". After checking the Help doc on webhooks, I found "st_diffs" in request body but is set as "null". Can anyone tell me why this field is set to null? Another question is, if I can get the diffs files of one merge request to master on jenkins? Thus I can filter out merge requests that don't need to run pipelines. Or is there any other ways to do this?