### Summary User lookup ajax call always returns users whose names consist of a single emoji symbol. ### Steps to reproduce 1. create a user on gitlab.com 2. create a private project 3. try to add an issue 4. the "Filter by Assignee" dropdown contains logged in user + a lot of unrelated usernames You can also login to gitlab.com and open the url: https://gitlab.com/autocomplete/users.json?search= which shows all users whose names consist of a single emoji. ### Example Project Not specific to any project. ### What is the current *bug* behavior? As described above. ### What is the expected *correct* behavior? Only the users that can access the private project should appear in the drop down. At least, there is no need for single-emoji-usernames to be sitting in that list. For the user lookup ajax call, if the "search" field is empty, and other parameters (like "current_user") are not specified, then do not return any result. ### Relevant logs and/or screenshots ### Output of checks This bug happens on GitLab.com