Validate not null constraint on projects.project_namespace_id column

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183649



Merged-by: Leonardo da Rosa <ldarosa@gitlab.com>
Approved-by: Irina Bronipolsky <ibronipolsky@gitlab.com>
Approved-by: Leonardo da Rosa <ldarosa@gitlab.com>
Reviewed-by: Leonardo da Rosa <ldarosa@gitlab.com>
Reviewed-by: Irina Bronipolsky <ibronipolsky@gitlab.com>
Co-authored-by: Abdul Wadood <awadood@gitlab.com>