Add unit tests for core/modules/newsletter #3464
Description
What is the motivation for adding/enhancing this feature?
Add unit tests for the module. Reference/example:
- https://github.com/DivanteLtd/vue-storefront/tree/develop/core/modules/catalog-next/test/unit
- https://github.com/DivanteLtd/vue-storefront/blob/develop/core/modules/url/test/unit/formatCategoryLink.spec.ts
What are the acceptance criteria
- all the actions should be tested
- all the helpers should be tested
- all the mutations should be tested