Error 500 when accessing autocomplete user list with anonymous user

Seen in https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/109721/

You can reproduce this easily:

curl -I "https://staging.gitlab.com/gitlab-org/gitlab-ce/autocomplete_sources/members"
HTTP/1.1 500 Internal Server Error

This might have been around for a while, but it seems like a silly bug.

/cc: @lbennett, @smcgivern