Fix update reviewers request when none is assigned

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/106306



Merged-by: Matthias Käppler <mkaeppler@gitlab.com>
Approved-by: Jay McCure <jmccure@gitlab.com>
Approved-by: Sashi Kumar Kumaresan <skumar@gitlab.com>
Approved-by: Matthias Käppler <mkaeppler@gitlab.com>
Co-authored-by: Tan Le <tle@gitlab.com>