It seems that gitlab changed the place of asignees in the payload.
At least in my tests they are not in the ObjectAttributes.Assignee
I had to add an array of Assignee to the MergeRequestEventPayload to get them.
see my diff
https://github.com/lsascha/webhooks/commit/122aaa16423b41a7cbc31f3d69c8a55f93ef0b2a.diff