[Feature] Add not null tests

### Is there an existing feature request for this?

- [X] 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](https://github.com/fivetran/dbt_stripe/blob/main/models/stripe.yml#L22-L28), 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_

Start with models/stripe.yml and the combination-of-columns test around lines 22-28, using balance_transaction_id as the example. Review the Stripe model definitions for other missing not_null coverage, then verify that the dbt tests pass; done means adding the appropriate tests wherever they are missing.

Written by the indexing model from the issue text.