Add NOT-NULL Constraint on ci_pipelines on project

https://docs.gitlab.com/ee/development/database/not_null_constraints.html#add-a-not-null-constraint-to-an-existing-column

We need to fix ci_pipelines to not allow null project values. This will enable sharding and unblock other tables that only have association to ci_pipelines

Corresponding MR: !155137 (merged)

Upon Part 2 of actually adding the NOT-NULL constraint, we need to change

validates project, on: :create to remove the on: :create as there should be no more invalid records