Mark primary keys in many-2-many tables as not null

[sverhoeven]

Most primary keys are marked as NOT NULL, but when they are used in many-2-many table as foreign keys they are not.

The docs says its mandatory like experiment_setup table must have measurement_pk and condition_pk.

Could NOT NULL be added to places where they are mandatory?

This will help my ORM generate correct Typescript types. Now I have to check if a pk value is null, while it never can be.

[imartorelli]

yes of course! thanks for noticing this.
I will update this for experiment_setup and chamber_describe