Sending null on Any milestones for board scope

What does this MR do and why?

This fixes a regression that happened when a user created a new board and clicked on the milestones scope dropdown. For Any, it should have sent null as the milestoneId instead of the id we use for "having a milestone."

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.