[Feature] Add not null tests

[fivetran-catfritz]

Copied from fivetran/dbt_stripe #97.

Is there an existing feature request for this?

• I have searched the existing issues

Describe the Feature

We should add not_null tests where appropriate. For example, we have a combination of columns test here, but we don't have a not_null test for balance_transaction_id.

How would you implement this feature?

Add not_null tests where they are missing.

Describe alternatives you've considered

No response

Are you interested in contributing this feature?

• Yes.
• Yes, but I will need assistance.
• No.

Anything else?

No response