Merge branch 'issuable-sidebar-dropdown-deselect-fix' into 'master'

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

2 files changed, 2 insertions, 2 deletions

diff --git a/app/assets/javascripts/milestone_select.js.coffee b/app/assets/javascripts/milestone_select.js.coffee
index 648e1f3bde0..b108f747bd6 100644
--- a/app/assets/javascripts/milestone_select.js.coffee
+++ b/app/assets/javascripts/milestone_select.js.coffee
@@ -116,7 +116,7 @@ class @MilestoneSelect
               .val()
             data = {}
             data[abilityName] = {}
-            data[abilityName].milestone_id = selected
+            data[abilityName].milestone_id = if selected? then selected else null
             $loading
               .fadeIn()
             $dropdown.trigger('loading.gl.dropdown')
diff --git a/app/assets/javascripts/users_select.js.coffee b/app/assets/javascripts/users_select.js.coffee
index 88246b0feb8..3dbc1d7f14f 100644
--- a/app/assets/javascripts/users_select.js.coffee
+++ b/app/assets/javascripts/users_select.js.coffee
@@ -31,7 +31,7 @@ class @UsersSelect
       assignTo = (selected) ->
         data = {}
         data[abilityName] = {}
-        data[abilityName].assignee_id = selected
+        data[abilityName].assignee_id = if selected? then selected else null
         $loading
           .fadeIn()
         $dropdown.trigger('loading.gl.dropdown')