Error 500 and other issues when searching for Merge Requests with author self and approver any

Summary

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/1951

Searching MR's with approvers has several bugs.

Steps to reproduce

When searching for a merge request on page: https://gitlab.com/dashboard/merge_requests

Issues:

1. On gitlab.com, select author: self. Try to select "any" for approver. It will switch a (random?) user. cf: https://www.youtube.com/watch?v=Tsz-qltjrNI

2. On our gitlab.mycompany.com: Select author: "self" and approver "any". Press return, and you've got a 500 error.

3. On our gitlab.mycompany.com: Only select "approver" and myself, press return. Page returns "Please select at least one filter to see results" which effectivly just renders this new feature useless as I just can't search for all MR's where I'm set as an approver.

Relevant logs and/or screenshots