Add unit tests for core/modules/mailer

[pkarw]

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

[krskibin]

I'll take this one. Can you assign me?

[pkarw]

@krskibin we can only assign issues to the core team members, but feel free to take this one; it's yours ;)