Merge branch 'validate-not-null-constraint-sharding-key-merge_request_reviewers' into 'master'

Validate NOT NULL on merge_request_reviewers sharding key

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182524



Merged-by: Brian Williams <bwilliams@gitlab.com>
Approved-by: Shubham Kumar <shukumar@gitlab.com>
Approved-by: Brian Williams <bwilliams@gitlab.com>
Co-authored-by: Shane Maglangit <smaglangit@gitlab.com>