Remove empty approvers

What does this MR do and why?

Remove empty approvers in regards to user_approvers_ids and group_approvers_ids.

Related issue: #354572 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable scan_result_policy feature flag (if you haven't pulled master for a while)
2. Create a security project as described in the docs
3. Create a new scan result policy with the project owner

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.