covariates: Finish making key

[gsrohde]

• [x] Fix rows with NULL trait_id
• [ ] Add not-NULL constraints

(trait_id, variable_id) is constrained to be unique, but the constituent columns should be constrained to be non-NULL.

Currently, 9 rows have NULL in the trait_id column.

The constraint to add are:

ALTER TABLE covariates ALTER COLUMN trait_id SET NOT NULL;
ALTER TABLE covariates ALTER COLUMN variable_id SET NOT NULL;

[dlebauer]

I removed the rows with trait_id = null

delete from covariates where trait_id is null