Merge requests can't be filtered by Any assignee

Summary

Setting Assignee to "Any Assignee" while looking at the list of open merge requests, gives an error. I use this often to look at all open merge requests.

Steps to reproduce

Go to the global merge request page Select "Any assignee"

What is the current bug behavior?

"Please select at least one filter to see results"

What is the expected correct behavior?

Show all open merge requests, this always worked like this

Gitlab version info:

This happens on gitlab.com but also on self-hosted instances, at least from 10.8.2-ee (we jumped from 10.7.x to 10.8.2)