Fixed issue with de-selecting dropdown option in issue sidebar

## What does this MR do?

When de-selecting either an assignee or milestone, we try to send `undefined` which jQuery removes from the request causing a routing error. Instead this MR sends `null` which jQuery still sends and then correctly removes either the milestone or assignee. 

## What are the relevant issue numbers?

Closes #18641 

## Screenshots (if relevant)

![assignee](/uploads/6eaca416fb6e31eabf4c038967160c07/assignee.gif)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4671