gitlabMergeRequestAssignee is null

### Before submitting an issue I have first:

- [X] read the documentation on the [homepage](https://github.com/jenkinsci/gitlab-plugin)
- [X] searched for similar already existing issue
- [X] confirmed that my GitLab webhook is using the http://JENKINS_URL/project endpoint and not the http://JENKINS_URL/job endpoint

*(if you have performed all the above, remove the paragraph and continue describing the issue with template below)*

## Issue

### Context
- **Gitlab plugin version**: 1.5.13
- **Gitlab version**: latest (gitlab.com)
- **Jenkins version**: 2.169
- **Job type**: Pipeline

### Problem description

I'm trying to get the variable **gitlabMergeRequestAssignee** in my pipeline but it is equal to null. Right now, other variables are accessible but not this one. Is it deprecated?
Thanks