Merge branch...

Merge branch 'c0rydoras/514167-validate-not-null-constraint-on-projects.project_namespace_id-column' into 'master' 

Validate not null constraint on projects.project_namespace_id column

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/178729



Merged-by: Krasimir Angelov <kangelov@gitlab.com>
Approved-by: Tianwen Chen <tchen@gitlab.com>
Approved-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Krasimir Angelov <kangelov@gitlab.com>
Reviewed-by: Tianwen Chen <tchen@gitlab.com>
Co-authored-by: Arthur Deierlein <arthur.deierlein@adfinis.com>