Add not null constraint on notes sharding key (NOT VALID)

Mario Celirequested to merge
444222-add-sharding-key-not-null-constraint-not-valid into master

What does this MR do and why?

We need to make sure at least one sharding key column is present in the notes sharding key before starting the backfill and we can validate the constraint.

The backfill will be scheduled in Backfill the namespace id for notes with null p... (!191155 - merged)

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Related to #444222 (closed)

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