Correctly handle nil assignee arguments

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/153404



Merged-by: Pavel Shutsin <pshutsin@gitlab.com>
Approved-by: Adam Hegyi <ahegyi@gitlab.com>
Approved-by: Pavel Shutsin <pshutsin@gitlab.com>
Co-authored-by: charlieablett <cablett@gitlab.com>