Merge branch '493768-sharding-key-not-null-workspace_variables' into 'master'

Add NOT NULL for sharding key on workspace_variables

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/177383



Merged-by: Omar Qunsul <oqunsul@gitlab.com>
Approved-by: Shubham Kumar <shukumar@gitlab.com>
Approved-by: Omar Qunsul <oqunsul@gitlab.com>
Co-authored-by: Shane Maglangit <smaglangit@gitlab.com>