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 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together